Large area nanoenabled macroelectronic substrates and uses therefor

ABSTRACT

A method and apparatus for an electronic substrate having a plurality of semiconductor devices is described. A thin film of nanowires is formed on a substrate. The thin film of nanowires is formed to have a sufficient density of nanowires to achieve an operational current level. A plurality of semiconductor regions are defined in the thin film of nanowires. Contacts are formed at the semiconductor device regions to thereby provide electrical connectivity to the plurality of semiconductor devices. Furthermore, various materials for fabricating nanowires, thin films including p-doped nanowires and n-doped nanowires, nanowire heterostructures, light emitting nanowire heterostructures, flow masks for positioning nanowires on substrates, nanowire spraying techniques for depositing nanowires, techniques for reducing or eliminating phonon scattering of electrons in nanowires, and techniques for reducing surface states in nanowires are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/940,789, filed Nov. 5, 2010, which is a division of U.S. patentapplication Ser. No. 11/760,382, filed Jun. 8, 2007, now U.S. Pat. No.7,851,841, which is a continuation of U.S. patent application Ser. No.11/490,637 filed Jul. 21, 2006, now U.S. Pat. No. 7,233,041, which is acontinuation of U.S. patent application Ser. No. 11/106,340 filed Apr.13, 2005, now U.S. Pat. No. 7,135,728, which is a continuation of U.S.patent application Ser. No. 10/674,060 filed Sep. 30, 2003, now U.S.Pat. No. 7,067,867, which claims priority to the following U.S.Provisional Applications, all of which are incorporated herein byreference in their entireties:

Provisional Application No. 60/414,323, filed Sep. 30, 2002;

Provisional Application No. 60/414,359, filed Sep. 30, 2002;

Provisional Application No. 60/445,421, filed Feb. 5, 2003;

Provisional Application No. 60/468,276, filed May 7, 2003;

Provisional Application No. 60/474,065, filed May 29, 2003; and

Provisional Application No. 60/488,801, filed Jul. 22, 2003.

The following application is related to the grandparent U.S. Pat. No.7,067,867, has the same filing date as the grandparent patent, and isherein incorporated by reference in its entirety:

“Integrated Displays Using Nanowire Transistors,” Ser. No. 10/673,669,now U.S. Pat. No. 7,102,605.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices, and moreparticularly, to the use of thin films of nanowires in semiconductordevices.

2. Background Art

An interest exists in industry in developing low cost electronics, andin particular, in developing low cost, large area electronic devices.Availability of such large area electronic devices could revolutionize avariety of technology areas, ranging from civil to militaryapplications. Example applications for such devices include drivingcircuitry for active matrix liquid crystal displays (LCDs) and othertypes of matrix displays, smart libraries, credit cards, radio-frequencyidentification tags for smart price and inventory tags, securityscreening/surveillance or highway traffic monitoring systems, large areasensor arrays, and the like.

The advancement of electronics has been moving towards two extremes interms of physical scale. Rapid miniaturization of microelectronicsaccording to Moore's law has led to increases in computing power whileat the same time enabling reductions in cost. At the same time, progresshas been made in the area of macroelectronics, in which electronicdevices are integrated over large area substrates (e.g., having sizesmeasured in square meters). Current macroelectronics are primarily basedon amorphous silicon (a-Si) or polycrystalline silicon (p-Si) thin filmtransistors (TFTs) on glass, and are finding important applications invarious areas, including flat panel display (FPD), solar cells, imagesensor arrays and digital x-ray imagers.

The current technology, however, is limited in what applications towhich it can be applied. For example, there has been growing interest inthe use of plastic as a substrate for macroelectronics due to variousbeneficial attributes of plastic, including flexibility, shockresistance, low weight, and low cost. However, the fabrication of highperformance TFTs on plastics is difficult because process steps must becarried out below the glass transition temperature of the plastic.Significant efforts have been devoted to search for new materials (suchas organics and organic-inorganic hybrids) or new fabrication strategiessuitable for TFTs on plastics, but only with limited success. OrganicTFTs have the potential for roll-to-roll fabrication process on plasticsubstrates, but with only a limited carrier mobility of about 1 cm²/V·s(centimeter squared per volt second). The limitations posed by materialsand/or substrate process temperature (particularly on plastic) lead tolow device performance, restricting devices to low-frequencyapplications. Therefore, applications that require even modestcomputation, control, or communication functions cannot be addressed bythe existing TFT technology.

Individual semiconductor nanowires (NWs) and single walled carbonnanotubes can be used to fabricate nanoscale field effect transistors(FETs) with electronic performance comparable to and in some caseexceeding that of the highest-quality single-crystal materials. Inparticular, carrier mobility of 300 cm²/V·s has been demonstrated forp-Si NWs, 2000-4000 cm²/V·s for n-indium InP NWs and up to 20,000cm²/V·s for single walled carbon nanotubes. These nano-FETs areextending Moore's law toward the molecular level. They are, however,currently difficult to implement for production-scale nanoelectronicsdue to the complexity and limited scalability of the device fabricationprocesses.

Accordingly, what is needed are higher performance conductive orsemiconductive materials and devices, and methods and systems forproducing lower-cost, high performance electronic devices andcomponents.

Furthermore, what is needed are high performance TFTs that can beapplied to plastics and other substrates requiring low processtemperatures.

What is also needed is a production scalable method for fabrication ofnanoscale semiconductor devices than can be used as high performanceTFTs.

BRIEF SUMMARY OF THE INVENTION

Methods, systems, and apparatuses for an electronic substrate having oneor more semiconductor devices formed thereon is described. A thin filmof semiconductor nanowires is formed on a substrate. The thin film ofnanowires is formed to have a sufficient density of nanowires to achievean operational current level. A plurality of semiconductor regions aredefined in the thin film of nanowires. Contacts are formed at thesemiconductor device regions to thereby provide electrical connectivityto the plurality of semiconductor devices.

In a first aspect of the present invention, a semiconductor device isformed. A plurality of nanowires are deposited onto a substrate in athin film. First and second electrical contacts are formed on thesubstrate. At least one of the nanowires couples the first electricalcontact to the second electrical contact. In aspects of the presentinvention, the deposited nanowires can be semiconducting, magnetic,ferroelectric, thermoelectric, piezoelectric, metallic or transitionmetal oxide nanowires.

In another aspect of the present invention, a thin film for use in oneor more semiconductor devices is fabricated. A first plurality ofnanowires that are p-doped are formed. A second plurality of nanowiresthat are n-doped are formed. The first plurality of nanowires and secondplurality of nanowires are deposited onto a substrate to form a thinfilm of nanowires that includes n-doped and p-doped nanowires. The thinfilm of nanowires exhibits characteristics of both n-doped and p-dopednanowires.

In another aspect of the present invention, an electrical device isformed incorporating nanowire heterostructures. A plurality of nanowiresare formed so that each nanowire has along its long axis at least onefirst portion doped with a first dopant and at least one second portiondoped with a second dopant. Each nanowire has a spacing betweenconsecutive junctions of the first and second portions substantiallyequal to a first distance. A pair of electrical contacts are formed onthe substrate. A distance between the electrical contacts isapproximately equal to the first distance. The plurality of nanowiresare deposited onto the substrate. At least one nanowire of the pluralityof nanowires couples the first electrical contact to the secondelectrical contact.

In another aspect of the present invention, a light emitting thin filmis fabricated incorporating nanowire heterostructures. At least onelight emitting semiconductor material. A plurality of nanowires areformed from the selected at least one light emitting semiconductormaterial. Each nanowire is doped so that each nanowire includes at leastone P-N junction. The plurality of nanowires are deposited onto asubstrate.

In still another aspect of the present invention, nanowires arepositioned on a target surface. A first surface of a flow mask is matedwith the target surface such that at least one channel formed in thefirst surface of the flow mask covers a portion of the target surface. Aliquid that contains a plurality of nanowires is flowed through the atleast one channel. Nanowires contained in the liquid flowing through theat least one channel are permitted to become positioned on the portionof the target surface covered by the at least one channel.

In still another aspect of the present invention, nanowires are appliedto a target surface. A solution source provides a nanowire solution. Thenanowire solution comprises a liquid containing a plurality ofnanowires. A nozzle is coupled to the solution source. The nozzle has atleast one output opening. The nozzle directs the nanowire solutionthrough the output opening(s) onto the target surface. The nanowires ofthe nanowire solution are directed onto the target surface to be alignedon said target surface substantially parallel to each other, or to berandomly oriented with respect to each other.

In still another aspect of the present invention, conducting nanowireshaving high mobility of electrons are designed. A semiconductor materialis selected. A maximum diameter for a nanowire made from the selectedsemiconductor material that provides substantial quantum confinement ofelectrons is determined.

In an example aspect, the diameter is determined by calculating themaximum diameter as follows:

${{the}\mspace{14mu}{maximum}\mspace{14mu}{diameter}} = \sqrt{\frac{{Nk}_{b}{T(8.9)}\hslash^{2}}{2\; m_{eff}}}$

wherein:

-   -   h=Planck's constant=4.14×10⁻¹⁵ eV-sec;    -   m_(eff)=effective mass of the selected semiconductor material;    -   N=a predetermined factor;    -   k_(b)=Boltzmann's constant=8.62×10⁻⁵ eV/° K; and    -   T=operating temperature;        -   wherein at room temperature, k_(b)T=0.0259 eV.

In still another aspect of the present invention, nanowires areconfigured to use electrons as conducting carriers to substantiallyreduce or entirely eliminate phonon scattering of electrons in thenanowires. In one aspect, the nanowires are doped with an n-type dopantmaterial to be configured to use electrons as conducting carriers. Inanother aspect, the nanowires are doped with an p-type dopant material.The nanowires are operated in an inversion mode by applying a sufficientbias voltage to a thin film of the nanowires so that electrons are usedas conducting carriers.

In still another aspect of the present invention, nanowires havingreduced surface scattering are fabricated. A semiconductor material isselected. A plurality of nanowires are formed from the selectedsemiconductor material. A circumferential surface of each nanowire ofthe plurality of nanowires is coated with an insulating layer.

In still another aspect of the present invention, nanowires havingreduced surface scattering are fabricated. A semiconductor material isselected. A plurality of nanowires are formed from the selectedsemiconductor material. Each nanowire of the plurality of nanowires isdoped so that each nanowire comprises a core-shell structure. The shellis a doped outer layer of each nanowire surrounding a respective core.Carriers of each nanowire are thereby caused to be substantiallyconfined to the core during operation.

In a further aspect, the present invention is directed to thin filmtransistors using nanowires, nanorods, or nanoribbons, and to productionscalable methods for producing such transistors on a variety ofsubstrates. In particular, an entirely new concept of macroelectronicshas been developed by using oriented semiconductor nanowire ornanoribbon thin films to produce thin film transistors (TFTs) with theconducting channel parallel to the wire/ribbon axis. These new TFTs havea conducting channel formed by multiple single crystal nanowires inparallel (like a log bridge) or a single crystal nanoribbon, whichcrosses all the way from source to drain electrode for high carriermobility.

In another aspect of the present invention, a NW-TFT fabrication methodis provided in which a high-temperature active semiconductor materialssynthesis process (e.g., used to form nanowires or nanoribbons) iscarried out before the active semiconductor materials are applied to adevice substrate. Subsequently, the formed NW-TFTs are applied to thedevice substrate via a solution assembly process, providing a generaltechnique for applying any semiconductor material to any substrate type,including a plastic substrate.

According to aspects of the invention, both p-channel and n-channel TFTscan be formed. In an example aspect, a complementary inverter isdescribed herein that is assembled from p-channel and n-channel TFTsusing a combination of nanowires and nanoribbons.

Systems and methods to further improve performance are described herein,according to further aspects of the present invention. For example,aspects of the present invention allow NW-TFT performance to match orexceed that of bulk single crystal materials. In an example aspect, byfabricating novel core-shell NW structures and fully exploiting quantumelectronic effects at reduced dimensions, carrier mobility can beenhanced to exceed that of bulk single crystal materials. In addition,approaches to fabricating NW-TFTs, according to aspects of the presentinvention, represent a general platform for a variety of macroelectronicapplications. In aspects of the present invention, NWs made of opticallyactive materials with various bandgaps are used to produce highperformance, optically active thin films for multiple color solid-statelight emitting diode (LED) displays. Furthermore, according to aspectsof the present invention, NW-TFTs can be deposited from solution ontolarge area substrates using low-cost, low-temperature processesincluding micro-contact or ink jet printing technology, for example.

In another aspect of this invention, electrical devices can be formedusing a structure that includes multiple nanowire thin film layers. Afirst plurality of nanowires are deposited on a substrate to form afirst nanowire thin film layer. A second plurality of nanowires aredeposited on the first nanowire thin film layer to form a secondnanowire thin film layer. Junctions, such as p-n junctions, are therebyformed at the cross points between the nanowires of the first and secondthin film layers. Contacts can be formed to create electrical devicesbased on the properties of the junctions. Nanowires of the first thinfilm layer are preferably aligned parallel to each other, and nanowiresof the second thin film layer are preferably aligned parallel to eachother. However, in alternative aspects, the nanowires of the firstand/or second thin film layers can be randomly oriented.

In another aspect of this invention, an electrical device is formed thatincludes a hybrid nanowire-single crystal semiconductor structure. Asingle-crystal semiconductor strip/thin film is formed. A plurality ofnanowires are deposited on the top of the strip. Junctions, such as p-njunctions, are formed at the cross points between the nanowires and thesingle crystal semiconductor strip. Contacts can be formed to createelectrical devices based on the properties of these junctions. Thenanowires of the plurality of nanowires are preferably aligned parallelto each other, but can alternatively be randomly oriented.

In another aspect of the present invention, an electrical device isformed that includes a hybrid nanowire-amorphous/polycrystallinesemiconductor structure. An amorphous or polycrystalline semiconductorthin film is deposited on a substrate. A plurality of nanowires aredeposited on the thin film pattern. Junctions, such as p-n junctions,are formed at the cross points between the nanowires and theamorphous/polycrystalline semiconductor thin film pattern. Contacts canbe formed to create electrical devices based on the properties of thesejunctions. The nanowires of the plurality of nanowires are preferablyaligned parallel to each other, but can alternatively be randomlyoriented.

In another aspect of the present invention, semiconductor nanowiresemitting red, green and blue light in a predetermined ratio can be mixedin a solution. The wire mixture is flowed across a single-crystal,amorphous, or polycrystalline semiconductor strip/thin film. Contactsare formed to create a light emitting electrical device. Depending onthe mixture of light emitting nanowires, any color of light can beemitted by the light emitting electrical device, including white light.

Thus, according to aspects of the present invention, nanowire, nanorod,nanoribbon, and nanotube thin films enable a variety of newcapabilities. In aspects, these include: moving microelectronics fromsingle crystal substrates to glass and plastic substrates; integratingmacroelectronics, microelectronics and nanoelectronics at the devicelevel; and, integrating different semiconductor materials on a singlesubstrate. These aspects of the present invention impact a broad rangeof existing applications, from flat-panel displays to image sensorarrays, and enable a whole new range of universal flexible, wearable,disposable electronics for computing, storage and communication.

These and other objects, advantages and features will become readilyapparent in view of the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 shows a view of a portion of a thin film of nanowires, accordingto an example embodiment of the present invention.

FIG. 2 shows a semiconductor device that includes a thin film ofnanowires, according to an example embodiment of the present invention.

FIGS. 3A-3D shows nanowires doped according to various exampleembodiments of the present invention.

FIGS. 4A and 4B show examples of a semiconductor device, doped accordingto example doping embodiments of the present invention.

FIG. 5 shows a flowchart providing example steps for making a pluralityof semiconductor devices, according to embodiments of the presentinvention.

FIGS. 6A-6F show various stages of fabrication for a substrate having aplurality of semiconductor devices thereon, according to an embodimentof the present invention.

FIG. 7 shows a flowchart providing example steps for making anelectrical device that incorporates a thin film of nanowires of thepresent invention, according to an embodiment of the present invention

FIG. 8A shows a close up view of an example portion of a nanowire thinfilm that includes a homogeneous mixture of n-doped nanowires andp-doped nanowires, according to an embodiment of the present invention.

FIG. 8B shows an example portion of a nanowire thin film that includesboth n-doped nanowires and p-doped nanowires.

FIG. 8C shows a thin film of nanowires that includes both n-dopednanowires and p-doped nanowires.

FIG. 9 shows a flowchart providing example steps for making a thin filmof nanowires of the present invention, according to an embodiment of thepresent invention.

FIG. 10 shows a nanowire that is a nanowire heterostructure, accordingto an example embodiment of the present invention.

FIG. 11A shows an example two-terminal electrical device that includes aplurality of nanowires, according to an embodiment of the presentinvention.

FIG. 11B shows an example p-n-p transistor, incorporating nanowireheterostructures.

FIG. 12 shows a flowchart providing example steps for making anelectrical device incorporating nanowire heterostructures, according toan embodiment of the present invention.

FIG. 13A shows a discrete pixel or light source, having a pair ofelectrodes, first electrical contact and second electrical contact,according to an embodiment of the present invention.

FIG. 13B shows a column of discrete pixels or light sources, eachsimilar to pixel or light source, according to an embodiment of thepresent invention.

FIG. 13C shows a large area light source, which includes a plurality oflight source columns, according to an embodiment of the presentinvention.

FIG. 14 shows a flowchart providing example steps for making a lightemitting device incorporating light emitting nanowire heterostructures,according to an embodiment of the present invention.

FIGS. 15A and 15B show bottom and cross-sectional views of an exampleflow mask, according to an embodiment of the present invention.

FIG. 16 shows a nanowire positioning system that incorporates flow mask,according to an example embodiment of the present invention.

FIGS. 17A and 17B show plan and cross-sectional views of a flow ofnanowires flowing through flow mask, according to an example embodimentof the present invention.

FIG. 18A shows an example semiconductor wafer mated with a flow mask,according to an embodiment of the present invention.

FIG. 18B shows portions of the surface of the wafer of FIG. 18A, havingnanowires positioned thereon, due to operation of the present invention.

FIG. 18C shows an array of integrated circuits formed on a wafer, withnanowires positioned thereon, due to operation of the present invention.

FIG. 19A shows an integrated circuit, which can be an example of one ofthe integrated circuits of the wafer shown in FIG. 18C, according to anembodiment of the present invention.

FIG. 19B shows a close-up view a portion of the integrated circuit ofFIG. 19A, showing detail of example electrically conductive traces,according to an embodiment of the present invention.

FIG. 19C shows nanowires having been deposited on the integrated circuitportion of FIG. 19B, by operation of an example flow mask of the presentinvention.

FIG. 19D shows an integrated circuit, which can be an example of one ofthe integrated circuits of the wafer shown in FIG. 18C, according to anembodiment of the present invention.

FIG. 19E shows a close-up view a portion of the integrated circuit ofFIG. 19D, showing detail of example electrically conductive traces,according to an embodiment of the present invention.

FIG. 19F shows nanowires having been deposited on the integrated circuitportion of FIG. 19E, by operation of an example flow mask of the presentinvention.

FIG. 20A shows a graph related to FIGS. 19A-C.

FIG. 20B shows a graph related to FIGS. 19D-19F.

FIG. 21 shows a flowchart providing example steps for positioningnanowires on a target surface using a flow mask, according to an exampleembodiment of the present invention.

FIG. 22 shows a block diagram of an example nanowire spray applicationsystem, according to an embodiment of the present invention.

FIG. 23 shows a detailed view of a nozzle outputting a flow of nanowiresonto an example target surface, according to an embodiment of thepresent invention.

FIGS. 24 and 25 show plan views of a target surfaces having a pluralityof nanowires positioned thereon, due to operation of the presentinvention.

FIG. 26 shows a plan view of a target surface with a plurality ofelectrical contacts formed thereon, in electrical contact withnanowires, according to an embodiment of the present invention.

FIG. 27 shows a flowchart providing example steps for positioningnanowires on a target surface using spray techniques, according to anexample embodiment of the present invention.

FIG. 28 shows a graph providing a relationship between a maximumallowable diameter for a semiconductor material and effective massm_(eff), according to an embodiment of the present invention.

FIG. 29 shows a table listing information about various examplesemiconductor materials.

FIG. 30 shows a flowchart providing example steps for designingconducting nanowires having high mobility of electrons, according to anexample embodiment of the present invention.

FIG. 31 shows a table listing information about example III-Vsemiconductor type materials.

FIGS. 32 and 33 show flowcharts providing example steps for fabricatingnanowires having reduced surface scattering, according to exampleembodiments of the present invention.

FIG. 34A is a diagram of amorphous or polycrystalline Si TFTs.

FIG. 34B is a diagram of a nanowire TFT, according to an embodiment ofthe invention.

FIG. 34C is a diagram of a nanoribbon TFT, according to an embodiment ofthe invention.

FIG. 35A is a flow chart of a method for NW-TFT fabrication, accordingto an embodiment of the invention.

FIG. 35B is a diagram of an optical micrograph of a NW thin film,according to an embodiment of the invention.

FIG. 35C is a diagram of a NW-TFT with gold electrodes, according to anembodiment of the invention.

FIG. 35D is a diagram of an optical micrograph of a NW-TFT with parallelarrays of NWs bridging from source to drain electrodes, according to anembodiment of the invention.

FIG. 36A is a chart showing typical drain current (I_(DS)) versusdrain-source bias voltage (V_(DS)) relations at different gate voltages(V_(GS)) in the steps of 1 volt (V) for a NW-TFT, according to anembodiment of the invention.

FIG. 36B is a chart showing a plot of I_(DS) versus V_(GS), for a NW-TFTaccording to an embodiment of the invention.

FIG. 36C is a chart showing a histogram of threshold voltagedistribution for a NW-TFT, according to an embodiment of the invention.

FIG. 36D is a chart illustrating the linear-scale relation for the draincurrent when the device is turned on (Vgs=−10V) for a NW-TFT, accordingto an embodiment of the invention.

FIG. 37A is a diagram of a NW-TFT on a plastic substrate, according toan embodiment of the invention.

FIG. 37B is a diagram of several NW-TFTs on plastic substrates,according to an embodiment of the invention.

FIG. 37C is a diagram showing drain current (I_(DS)) versus drain-sourcebias voltage (V_(DS)) relations at different gate voltages (V_(GS)) inthe steps of 1 volt (V) for a NW-TFT on a plastic substrate, accordingto an embodiment of the invention.

FIG. 37D is a diagram showing the transfer characteristics of the sameNW-TFT before and after slight flexing of the plastic substrate,according to an embodiment of the invention.

FIG. 38A is a diagram of a NW-TFT on a plastic substrate with anelectrolyte solution gate, according to an embodiment of the invention.

FIG. 38B is a chart of the I_(DS)−V_(DS) relation as a function ofvarious electrolyte solution gate voltages for a NW-TFT on a plasticsubstrate, according to an embodiment of the invention.

FIG. 38C is a chart of the I_(DS)−V_(GS) relation for a V_(DS) of 10 mVfor a NW-TFT on a plastic substrate with an electrolyte solution gate,according to an embodiment of the invention.

FIG. 39A is a diagram of a CdS nanoribbon TFT, according to anembodiment of the invention.

FIG. 39B is a chart of the I_(DS)−V_(DS) relation as a function ofvarious gate voltages for a CdS nanoribbon TFT, according to anembodiment of the invention.

FIG. 39C is a chart of the I_(DS)−V_(GS) relation with a V_(DS) of 1 Vfor a CdS nanoribbon TFT, according to an embodiment of the invention.

FIG. 40 is a diagram of a complementary inverter made with a p-channelNW-TFT and an n-channel CdS nanoribbon TFT along with gaincharacteristics, according to an embodiment of the invention.

FIG. 41A shows a scanning electron microscope image of synthesizedsilicon nanowires, according to an example embodiment of the presentinvention.

FIG. 41B shows a lattice-resolved transmission electron microscope imageof individual Si nanowires, according to an example embodiment of thepresent invention.

FIG. 42 shows a flow diagram of a process for synthesizing andimplementing high mobility nanowire thin film transistors, according toan example embodiment of the present invention.

FIG. 43 shows a silicon nanowire core-shell structure with a singlecrystalline core and dielectric overcoating, according to an exampleembodiment of the present invention.

FIGS. 44A-C shows schematic views of thin film transistors (TFTs)fabricated from amorphous silicon, polysilicon, and an aligned nanowirethin film.

FIG. 45 shows a diagram of a fluidic cell for aligning nanowires over alarge area, according to an embodiment of the present invention.

FIG. 46 shows a diagram illustrating the alignment of nanowires over alarge area using a Langmuir-Blodgett film, according to an exampleembodiment of the present invention.

FIG. 47 shows plan and perspective views of a single nanowire fieldeffect transistor, according to an example embodiment of the presentinvention.

FIGS. 48A and 48B show perspective views of locally gated nanowire thinfilm transistors, according to embodiments of the present invention.

FIG. 49 schematically illustrates an alternative embodiment of asubstrate having a plurality of semiconductor devices thereon.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION OF THE INVENTION Introduction

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanowire (NW), nanorod, nanotube, andnanoribbon technologies and other functional aspects of the systems (andcomponents of the individual operating components of the systems) maynot be described in detail herein. Furthermore, for purposes of brevity,the invention is frequently described herein as pertaining to nanowires,and to a semiconductor transistor device. Moreover, while the number ofnanowires and spacing of those nanowires are provided for the specificimplementations discussed, the implementations are not intended to belimiting and a wide range of the number of nanowires and spacing canalso be used. It should be appreciated that although nanowires arefrequently referred to, the techniques described herein are alsoapplicable to nanorods, nanotubes, and nanoribbons. It should further beappreciated that the manufacturing techniques described herein could beused to create any semiconductor device type, and other electroniccomponent types. Further, the techniques would be suitable forapplication in electrical systems, optical systems, consumerelectronics, industrial electronics, wireless systems, spaceapplications, or any other application.

As used herein, the term “nanowire” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) that includes at least one cross sectional dimension that isless than 500 nm, and preferably, less than 100 nm, and has an aspectratio (length:width) of greater than 10, preferably, greater than 50,and more preferably, greater than 100. Examples of such nanowiresinclude semiconductor nanowires as described in Published InternationalPatent Application Nos. WO 02/17362, WO 02/48701, and 01/03208, carbonnanotubes, and other elongated conductive or semiconductive structuresof like dimensions.

As used herein, the term “nanorod” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) similar to a nanowire, but having an aspect ratio (length:width)less than that of a nanowire. Note that two or more nanorods can becoupled together along their longitudinal axis so that the couplednanorods span all the way between electrodes. Alternatively, two or morenanorods can be substantially aligned along their longitudinal axis, butnot coupled together, such that a small gap exists between the ends ofthe two or more nanorods. In this case, electrons can flow from onenanorod to another by hopping from one nanorod to another to traversethe small gap. The two or more nanorods can be substantially aligned,such that they form a path by which electrons can travel betweenelectrodes.

While the example implementations described herein principally use CdSand Si, other types of materials for nanowires and nanoribbons can beused, including semiconductive nanowires or nanoribbons, that arecomprised of semiconductor material selected from, e.g., Si, Ge, Sn, Se,Te, B, C (including diamond), P, B—C, B—P(BP6), B—Si, Si—C, Si—Ge, Si—Snand Ge—Sn, SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb,InN/InP/InAs/InSb, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb,InN/InP/InAs/InSb, ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe,BeS/BeSe/BeTe/MgS/MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS,PbSe, PbTe, CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, BeSiN2, CaCN₂,ZnGeP₂, CdSnAs₂, ZnSnSb₂, CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl,Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO,and an appropriate combination of two or more such semiconductors.

In certain aspects, the semiconductor may comprise a dopant from a groupconsisting of: a p-type dopant from Group III of the periodic table; ann-type dopant from Group V of the periodic table; a p-type dopantselected from a group consisting of: B, Al and In; an n-type dopantselected from a group consisting of: P, As and Sb; a p-type dopant fromGroup II of the periodic table; a p-type dopant selected from a groupconsisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of theperiodic table; a p-type dopant selected from a group consisting of: Cand Si; or an n-type is selected from a group consisting of: Si, Ge, Sn,S, Se and Te.

Additionally, the nanowires or nanoribbons can include carbon nanotubes,or nanotubes formed of conductive or semiconductive organic polymermaterials, (e.g., pentacene, and transition metal oxides).

Hence, although the term “nanowire” is referred to throughout thedescription herein for illustrative purposes, it is intended that thedescription herein also encompass the use of nanotubes (e.g.,nanowire-like structures having a hollow tube formed axiallytherethrough). Nanotubes can be formed in combinations/thin films ofnanotubes as is described herein for nanowires, alone or in combinationwith nanowires, to provide the properties and advantages describedherein.

Furthermore, it is noted that a thin film of nanowires of the presentinvention can be a “heterogeneous” film, which incorporatessemiconductor nanowires and/or nanotubes, and/or nanorods, and/ornanoribbons, and/or any combination thereof of different compositionand/or structural characteristics. For example, a “heterogeneous film”can includes nanowires/nanotubes with varying diameters and lengths, andnanotubes and/or nanotubes that are “heterostructures” having varyingcharacteristics.

In the context of the invention, although the focus of the detaileddescription relates to use of nanowire, nanorod, nanotube, or nanoribbonthin films on plastic substrates, the substrate to which these nanostructures are attached may comprise other materials, including, but notlimited to: a uniform substrate, e.g., a wafer of solid material, suchas silicon, glass, quartz, polymerics, etc.; a large rigid sheet ofsolid materials, e.g., glass, quartz, plastics such as polycarbonate,polystyrene, etc., or can comprise additional elements, e.g.,structural, compositional, etc. A flexible substrate, such as a roll ofplastic such as polyolefins, polyamide, and others, a transparentsubstrate, or combinations of these features can be employed. Forexample, the substrate may include other circuit or structural elementsthat are part of the ultimately desired device. Particular examples ofsuch elements include electrical circuit elements such as electricalcontacts, other wires or conductive paths, including nanowires or othernanoscale conducting elements, optical and/or optoelectrical elements(e.g., lasers, LEDs, etc.), and structural elements (e.g.,microcantilevers, pits, wells, posts, etc.).

By substantially “aligned” or “oriented” is meant that the longitudinalaxes of a majority of nanowires in a collection or population ofnanowires is oriented within 30 degrees of a single direction. Althoughthe majority can be considered to be a number of nanowires greater than50%, in various embodiments, 60%, 75%, 80%, 90%, or other percentage ofnanowires can be considered to be a majority that are so oriented. Incertain preferred aspects, the majority of nanowires are oriented within10 degrees of the desired direction. In additional embodiments, themajority of nanowires may be oriented within other numbers or ranges ofdegrees of the desired direction, including randomly oriented andisotropically oriented.

It should be understood that the spatial descriptions (e.g., “above”,“below”, “up”, “down”, “top”, “bottom”, etc.) made herein are forpurposes of illustration only, and that devices of the present inventioncan be spatially arranged in any orientation or manner.

The materials used for nanowires described herein also have an inherentmechanical flexibility of the high-mobility semiconductor material,allowing fabrication of truly flexible high-performance electronics. Dueto the extremely small diameter and large aspect ratio (in someembodiments >1,000), nanowires possess superior mechanical flexibilityand strength. Individual nanowires can easily bend with radius ofcurvature r<10 μm before failure. Because each individual nanowire onthese high-density substrates is aligned in the same direction, butphysically independent of the surrounding wires, this flexibility isretained in the nanowire thin films of the present invention, includingdense, inorganic and oriented nanowire thin-films (DION thin-films).Even without bending the individual nanowires within a device, the factthat each nanowire is only 100 μm long allows a macroscopic r<<1 mm.

Thin Films of Nanowires Embodiments

The present invention is directed to the use of nanowires in systems anddevices to improve system and device performance. For example, thepresent invention is directed to the use of nanowires in semiconductordevices. According to the present invention, multiple nanowires areformed into a high mobility thin film. The thin film of nanowires isused in electronic devices to enhance the performance andmanufacturability of the devices.

FIG. 1 shows a close-up view of a thin film of nanowires 100, accordingto an example embodiment of the present invention. Thin film ofsemiconductor nanowires 100 can be used instead of amorphous silicon ororganic thin films in conventional electronic devices to achieveimproved device behavior, while allowing for a straight forward andinexpensive manufacturing process. Through the use of thin films ofnanowires, the present invention is particularly adapted to making highperformance, low cost devices on large and flexible substrates.

Note that thin film of nanowires 100 as described herein may be formedin a wide range of possible surface areas. For example, thin films ofnanowires 100 of the present invention can be formed to have functionalareas greater than 1 mm², greater than 1 cm², greater than 10 cm²,greater than 1 m², and even greater or smaller areas.

As shown in FIG. 1, thin film of nanowires 100 includes a plurality ofindividual nanowires closely located together. Thin film of nanowires100 can have a variety of thickness amounts that are equal to or greaterthan the thickness of a single nanowire. In the example of FIG. 1, thenanowires of thin film of nanowires 100 are aligned such that their longaxes are substantially parallel to each other. Note that in alternativeembodiments, the nanowires of thin film of nanowires 100 are notaligned, and instead can be oriented in different directions withrespect to each other, either randomly or otherwise. In an alternativeembodiment, the nanowires of thin film of nanowires 100 may beisotropically oriented, so that high mobility is provided in alldirections. Note that the nanowires of thin film of nanowires 100 may bealigned in any manner relative to the direction of electron flow inorder to enhance performance as required by a particular application.

FIG. 2 shows a semiconductor device 200 that includes thin film ofnanowires 100, according to an example embodiment of the presentinvention. In FIG. 2, semiconductor device 200 is shown as a transistor,having a source electrode 202, a gate electrode 204, a drain electrode206, formed on a substrate 208. Thin film of nanowires 100 is coupledbetween source electrode 202 and drain electrode 206 over a portion ofgate electrode 204. Thin film of nanowires 100 substantially operates asa channel region for the transistor of semiconductor device 200, andallows semiconductor 200 to operate with enhanced characteristics, asfurther described herein. Numerous substrate types applicable tosubstrate 208 are described elsewhere herein.

Note that semiconductor device 200 is shown as a transistor in FIG. 2for illustrative purposes. It would be understood to persons skilled inthe relevant art(s) from the teachings herein that thin film ofnanowires 100 can be included in semiconductor device types in additionto transistors, including diodes.

In embodiments, the nanowires of thin film of nanowires 100 are singlecrystal semiconductor nanowires that span all the way between sourceelectrode 202 and drain electrode 206. Hence, electric carriers cantransport through the single crystals nanowires, resulting in highmobility which is virtually impossible to obtain with current amorphousand polysilicon technologies.

As described above, the nanowires of thin film of nanowires 100 can bealigned or oriented. For example, the nanowires of thin film ofnanowires 100 shown in FIG. 2 can be aligned parallel to the length ofthe channel between source electrode 202 and drain electrode 206, or canbe aligned in alternative ways.

Thin film of nanowires 100 can be formed with a sufficient number ofnanowires to provide desired characteristics for semiconductor device200. For example, thin film of nanowires 100 can be formed of asufficient number of nanowires to achieve a desired current density orcurrent level desired for the particular semiconductor device. Forinstance, in the transistor example of FIG. 2, thin film of nanowires100 can be formed to have a current level in the channel of greater thanabout 10 nanoamps.

In an embodiment, a thin film of nanowires 100 can be formed to haveasymmetric mobility. For example, this can be accomplished byasymmetrically aligning the nanowires of thin film of nanowires 100,and/or by doping the nanowires in a particular manner. Such asymmetricmobility can be caused to be much greater in a first direction than in asecond direction. For example, asymmetric mobilities can be created inthe order of 10, 100, 1000, and 10000 times greater in the firstdirection than in the second direction, or to have any other asymmetricmobility ratio between, greater, or less than these values.

The nanowires of thin film of nanowires 100 can be doped in various waysto improve performance. The nanowires can be doped prior to inclusion insemiconductor device 200, or after inclusion. Furthermore, a nanowirecan be doped differently along portions of its long axis, and can bedoped differently from other nanowires in thin film of nanowires 100.Some examples of doping schemes for individual nanowires, and for thinfilms of nanowires are provided as follows. However, it will be apparentto persons skilled in the relevant art(s) from the teachings herein thatnanowires, and thin films thereof, can be doped according to additionalways, and in any combination of the ways described herein.

FIG. 3A shows a nanowire 300 that is a uniformly doped single crystalnanowire. Such single crystal nanowires can be doped into either p- orn-type semiconductors in a fairly controlled way. Doped nanowires suchas nanowire 300 exhibit improved electronic properties. For instance,such nanowires can be doped to have carrier mobility levels comparableto alternative single crystal materials. In addition, and without beingbound to any particular theory of operation, due to a one-dimensionalnature of the electron-wave traversing inside the nanowire channel, anda reduced scattering probability, it may be possible for such nanowiresto achieve even higher mobility than a bulk single crystal material.Carrier mobility levels up to 1500 cm²/V·s have been shown for singlep-type Si (silicon) nanowires, and carrier mobility levels up to 4000cm²/V·s have been shown for n-type InP nanowires.

FIG. 3B shows a nanowire 310 doped according to a core-shell structure.As shown in FIG. 3B, nanowire 310 has a doped surface layer 302, whichcan have varying thickness levels, including being only a molecularmonolayer on the surface of nanowire 310. Such surface doping canseparate impurities from a conducting channel of the nanowire, andsuppress an impurity-related scattering event, and thus may lead togreatly enhanced carrier mobility. For example, when nanowires are dopedaccording to the core-shell structure, “ballistic” transport may beachieved inside the nanowires. “Ballistic” transport is where electricalcarriers are transported through a nanowire with essentially noresistance. Further detail on doping of nanowires is provided below.

FIG. 3C shows a nanowire 320 that is uniformly doped, and coated with adielectric material layer 304, according to another type of core-shellstructure. Dielectric material layer 304 can be chosen from a variety ofdielectric materials, such as SiO₂ or Si₃N₄. The use of dielectricmaterial layer 304 can simplify fabrication of semiconductor device 200,as described elsewhere herein. Dielectric material layer 304 can beformed on nanowire 320, as is further described below.

FIG. 3D shows a nanowire 330 that is doped with a doped surface layer302 according to the core-shell structure shown in FIG. 3B, and is alsocoated with a dielectric material layer 304, as shown in FIG. 3C.

FIGS. 4A and 4B show examples of semiconductor device 200, according toexample doping embodiments of the present invention. As shown in FIG.4A, the top surface of substrate 208 is coated with a dopant layer 402.Dopant layer 402 includes electron-donor or electron acceptor dopingmaterials. Properties of semiconductor device 200 can be controlled bythe introduction of dopant layer 402. The electron-donor or electronacceptor materials introduce negative or positive charge carriers intothe nanowires to achieve n- or p-channel transistors, respectively. Veryhigh mobility levels can be attained in this configuration forsemiconductor device 200 because the dopants are separated from theactual conducting channel.

As shown in FIG. 4B, dopant layer 402 covers a region of substrate 208substantially localized around thin film of nanowires 100. Inembodiments, dopant layer 402 applied to semiconductor device 200 can bepatterned to have two or more areas doped according to different n- andp-type characteristics. For example, in the embodiment of FIG. 4B,dopant layer 402 has a first portion 404 doped with an n-typecharacteristic, and a second portion 406 doped with a p-typecharacteristic. In such an embodiment, a p-n junction can be achievedaccording to a variety of electronic and optoelectronic devices,including light-emitting diodes (LEDs).

As described above, dopant layer 402 can be introduced on substrate 208prior to or after actual fabrication of semiconductor device 200.

Collections of nanowires manufactured with these materials are usefulbuilding blocks for high performance electronics. A collection ofnanowires orientated in substantially the same direction will have ahigh mobility value. Furthermore, nanowires can be flexibly processed insolution to allow for inexpensive manufacture. Collections of nanowirescan be easily assembled onto any type of substrate from solution toachieve a thin film of nanowires. For example a thin film of nanowiresused in a semiconductor device can be formed to include 2, 5, 10, 100,and any other number of nanowires between or greater than these amounts,for use in high performance electronics.

Note that nanowires can also be used to make high performance compositematerials when combined with polymers/materials such as organicsemiconductor materials, which can be flexibly spin-cast on any type ofsubstrate. Nanowire/polymer composites can provide properties superiorto a pure polymer materials. Further detail on nanowire/polymercomposites is provided below.

As described above, collections or thin films of nanowires can bealigned into being substantially parallel to each other, or can be leftnon-aligned or random. Non-aligned collections or thin films ofnanowires provide electronic properties comparable or superior topolysilicon materials, which typically have mobility values in the rangeof 1-10 cm²/V·s.

Aligned collections or thin films of nanowires provide for materialshaving performance comparable or superior to single crystal materials.Furthermore, collections or thin films of nanowires that include alignedballistic nanowires (e.g., core-shell nanowires as shown in FIG. 3B) canprovide dramatically improved performance over single crystal materials.

Aligned and non-aligned, and composite and non-composite thin films ofnanowires can be produced in a variety of ways, according to the presentinvention. Example embodiments for the assembly and production of thesetypes of thin films of nanowires are provided as follows.

Randomly oriented thin films of nanowires can be obtained in a varietyof ways. For example, nanowires can be dispersed into a suitablesolution. The nanowires can then be deposited onto a desired substrateusing spin-casting, drop-and-dry, flood-and-dry, or dip-and-dryapproaches. These processes can be undertaken multiple times to ensure ahigh degree of coverage. Randomly oriented thin films ofnanowires/polymer composites can be produced in a similar way, providingthat the solution in which the nanowires are dispersed is a polymersolution.

Aligned thin films of nanowires can be obtained in a variety of ways.For example, aligned thin films of nanowires can be produced by usingthe following techniques: (a) Langmuir-Blodgett film alignment; (b)fluidic flow approaches, such as described in U.S. Pat. No. 6,872,645,and incorporated herein by reference in its entirety; and (c)application of mechanical shear force. For example, mechanical shearforce can be used by placing the nanowires between first and secondsurfaces, and then moving the first and second surfaces in oppositedirections to align the nanowires. Aligned thin films ofnanowires/polymer composites can be obtained using these techniques,followed by a spin-casting of the desired polymer onto the created thinfilm of nanowires. For example, nanowires may be deposited in a liquidpolymer solution, alignment can then be performed according to one ofthese or other alignment processes, and the aligned nanowires can thenbe cured (e.g., UV cured, crosslinked, etc.). An aligned thin film ofnanowires/polymer composite can also be obtained by mechanicallystretching a randomly oriented thin film of nanowires/polymer composite.

Fabrication of Electronic Devices Incorporating Thin Films of Nanowires

Embodiments for making electronic devices and systems that incorporatethin films of nanowires according to the present invention are furtherdescribed in this subsection. These implementations are described hereinfor illustrative purposes, and are not limiting. The electronic devicesand systems of the present invention, as described in this section, canbe fabricated in alternative ways, as would be apparent to personsskilled in the relevant art(s) from the teachings herein.

FIG. 5 shows a flowchart 500 providing example steps for making aplurality of semiconductor devices that incorporate the thin film ofnanowires of the present invention. The steps of FIG. 5 do notnecessarily have to occur in the order shown, as will be apparent topersons skilled in the relevant art(s) based on the teachings herein.Other structural embodiments will be apparent to persons skilled in therelevant art(s) based on the following discussion. These steps aredescribed in detail below.

Flowchart 500 begins with step 502. In step 502, a thin film ofnanowires is formed on a substrate with a sufficient density ofnanowires to achieve an operational current level. For example, asdescribed above, a thin film of nanowires can be formed on a substratein a variety of ways, according to the present invention. The thin filmof nanowires is formed with a sufficient density of nanowires to achievean operational current level. A sufficient operational current level istypically determined on an application basis. For example, the currentlevel may be in the nanoamp range, including 2 nanoamps, and greater andlesser current levels. The thin film of nanowires can be formed invarious ways described elsewhere herein to obtain the requiredoperational current level. The thin film of nanowires can be aligned ornon-aligned, and can be a composite or non-composite.

For example, to achieve the required operational current density, aminimum number of nanowires can be included in the thin film ofnanowires for a given area on the substrate. Hence, each formedsemiconductor device will have a sufficient number of nanowires to carrycurrent at an operational current level. For example, the requirednumber of nanowires per unit area can be 1 nanowire, 2 nanowires, andany other greater number of nanowires, including 5, 10, 100 or more.

In step 504, a plurality of semiconductor device regions are defined inthe thin film of nanowires. For example, referring to the singlesemiconductor device 200 shown in FIG. 2, the overall thin film ofnanowires formed in step 502 is patterned to form a localized thin filmof nanowires 100 for each region of the substrate where a semiconductordevice is being formed. In alternative embodiments, the thin film ofnanowires does not need to be patterned. Note that on a particularsubstrate, the semiconductor device regions can all define the samesemiconductor device type, or can define two or more differentsemiconductor device types.

In step 506, contact (e.g., electrode) areas are formed at thesemiconductor device regions to thereby provide electrical connectivityto the plurality of semiconductor devices. A semiconductor device canhave any number of required contact areas formed to provide electricalconnectivity. For example, a diode or other two terminal device can haveanode and cathode electrodes formed. For example, again referring to thesingle semiconductor device 200 shown in FIG. 2, three contact areaswere formed: source electrode 202, gate electrode 204, and drainelectrode 206. Other semiconductor devices can have greater or fewernumbers of contact areas formed.

Note that a variety of contact area types can be formed in step 506. Thecontact areas can be Ohmic and non-Ohmic. For example, a non-OhmicSchottky diode barrier contact can be used as an electrode. A Schottkydiode barrier contact is commonly used for a III-V semiconductormaterial when it is difficult to make a high quality gate dielectrics.Source electrodes 202, gate electrodes 204, and drain electrodes 206 areformed of a conductive material, such as a metal, alloy, silicide,polysilicon, or the like, including combinations thereof, as would beapparent to a person having ordinary skill in the art.

In some embodiments, flowchart 500 of FIG. 5 can include the step ofdoping the nanowires. The nanowires can be doped prior to being formedinto a thin film, or after being formed into a thin film. A thin film ofnanowires can be doped after being formed on the substrate. Thenanowires can be doped in numerous ways, including those ways describedabove with respect to FIGS. 3A-3D. For example, a core of a nanowire canbe doped and/or a shell layer of a nanowire can be doped. Furthermore,the individual nanowires and/or the thin film of nanowires can be dopeddifferently in different areas along their respective lengths.

Furthermore, in some embodiments, flowchart 500 of FIG. 5 can includethe step of forming a dielectric layer on the nanowires. The dielectriclayer can be formed by oxidizing the nanowires, or otherwise forming thedielectric layer. For example, other non-oxided high dielectric constantmaterials can be used, including silicon nitride, Ta₂O₅, TiO₂, ZrO₂,HfO₂, Al₂O₃, and others. Nitridation of nanowires can be accomplishedwith processes similar to those employed in oxidation of nanowires.These materials can be applied to nanowires by chemical vapor deposition(CVD), solution phase over-coating, or simply by spin-coating theappropriate precursor onto the substrate. Other known techniques can beemployed.

The steps of FIG. 5 are adaptable to fabrication of single or multiplesemiconductor devices on a substrate. FIGS. 6A-6F show various stages offabrication for a substrate 600 having a plurality of semiconductordevices thereon, according to an embodiment of the present invention.The steps of flowchart 500 shown in FIG. 5 are mentioned below inrelation to the processes shown in FIGS. 6A-6F.

FIG. 6A shows a perspective view of substrate 600. Substrate 600 can beany substrate type, including silicon, glass, quartz, polymeric, and anyother substrate type describe herein or otherwise known. Substrate 600can be large area or small area, and can be rigid or flexible, such as aflexible plastic or thin film substrate type. Substrate 600 can beopaque or transparent, and can be made from a conductive,semiconductive, or a non-conductive material.

FIG. 6B shows the patterning of a plurality of gates 204 on substrate600. Gates 204 can be patterned on substrate 600 using standardphotolithography, ink-jet printing, or micro-contact printing processes,for example, or by other processes. The patterning of plurality of gates204 shown in FIG. 6B can be performed during step 506 of flowchart 500shown in FIG. 5, for example.

FIG. 6C shows deposition of a dielectric layer 602 on substrate 600.Dielectric layer 602 electrically insulates the plurality of gates 204.The deposition of dielectric layer 602 on substrate 600 can be doneusing evaporation, solution cast of polymer or oxide dielectrics, and byother processes. Note that the deposition of dielectric layer 602 onsubstrate 600 may not be necessary if the nanowires to be deposited onsubstrate 600 are insulated by their own dielectric layer. For example,nanowires 320 and 330 shown in FIGS. 3C and 3D have a dielectricmaterial layer 304 pre-formed on their surfaces. Direct contact deviceswithout a dielectric layer are also envisioned.

FIG. 6D shows deposition of a thin film of nanowires 604 on substrate600. Deposition of thin film of nanowires 604 can be done using variousprocedures described elsewhere herein, including spin-casting,Langmuir-Blodgett alignment, mechanical alignment, and flow-alignmenttechniques. The deposition of the thin film of nanowires 604 shown inFIG. 6D can be performed during step 502 of flowchart 500 shown in FIG.5, for example.

FIG. 6E shows patterning of thin film of nanowires 604 into a pluralityof thin films of nanowires 100. Patterning of thin film of nanowires 604can be done using various processes, including lithography techniques.Note that deposition and patterning of thin film of nanowires 604, asshown in FIGS. 6D and 6E, can be done simultaneously using variousprocesses, such as inkjet printing or micro-contact printing methods.The patterning of plurality of thin film of nanowires 604 into pluralityof thin films of nanowires 100 shown in FIG. 6E can be performed duringstep 504 of flowchart 500 shown in FIG. 5, for example.

FIG. 6F shows patterning of a plurality of sources 202 and a pluralityof drains 206 on substrate 600, to form a plurality of semiconductordevices 200. Source and drain electrode patterning can be done usingprocesses similar to those used to pattern the plurality of gates 204,as shown in FIG. 6B. The patterning of the plurality of sources anddrains shown in FIG. 6F can be performed during step 506 of flowchart500 shown in FIG. 5, for example.

Note that the order in which the gates 204, sources 202, and drains 206are patterned can be varied. For example, gates 204, sources 202, anddrains 206 can be patterned simultaneously with each other or atdifferent times. They can be all be patterned prior to deposition of thethin film of nanowires 604, or afterwards. Sources 202 and drains 206can be patterned prior to deposition of the thin film of nanowires 604,while gates 204 are patterned afterwards. Alternatively, gates 204 canbe patterned prior to deposition of the thin film of nanowires 604,while sources 202 and drains 206 are patterned afterwards. Either ofsources 202 and drains 206 can also be patterned prior to deposition ofthe thin film of nanowires 604, while the other is patterned afterwards.

Note that in some embodiments, more than one layer of a thin film ofnanowires can be applied to a substrate in a given area. The multiplelayers can allow for greater electrical conductivity, and can be used tomodify electrical characteristics of a respective semiconductor device.The multiple layers can be similar, or different from each other. Forexample, two or more layers of thin films of nanowires having nanowiresaligned in different directions, doped differently, and/or differentlyinsulated, can be used in a particular semiconductor device. A contactarea of a particular semiconductor device can be coupled to any one ormore of the layers of a multiple layer thin film of nanowires. Note thata thin film of nanowires can be formed as a monolayer of nanowires, asub-monolayer of nanowires, and greater than a monolayer of nanowires,as desired.

Large Area Macroelectronic Substrate Nanowire Materials

As described above, electronic and electro-optic films can be formed onmacroelectronic (i.e., large area electronic) substrates using nanowiresmade from semiconducting materials. Furthermore, according toembodiments of the present invention, nanowires can be made fromalternative materials, and these nanowires can be used to form films ina similar manner as for nanowires made from semiconductor materials.

In embodiment of the present invention, nanowires can be made frommaterials such as magnetic materials, ferroelectric materials,thermoelectric materials, piezoelectric materials, metal(s)/alloys, andtransition metal oxide materials. Furthermore, corresponding thin filmscan be formed from the magnetic nanowires, ferroelectric nanowires,thermoelectric nanowires, piezoelectric nanowires, metal(s)/alloys, andtransition metal oxide nanowires. These thin films therefore exhibitproperties of the corresponding magnetic, ferroelectric, thermoelectric,piezoelectric, metallic, or transition metal oxide materials, and areable to be formed on large area substrates, which can be flexible ornon-flexible. Thus, entirely new materials/devices can be formed,according to the present invention.

For example, nanowire 300 shown in FIG. 3A can be a nanowire made from amagnetic, ferroelectric, thermoelectric, piezoelectric, a metallic, ortransition metal oxide material. Furthermore, thin film of nanowires 100which is shown in FIG. 1, for example, can therefore be a thin film ofmagnetic nanowires, a thin film of ferroelectric nanowires, a thin filmof thermoelectric nanowires, a thin film of piezoelectric nanowires, athin film of metallic nanowires, or a thin film of transition metaloxide nanowires, or any combination thereof.

Thus, devices, such as semiconductor device 200 shown in FIG. 2, can beformed using thin films of nanowires made from one or more of thesematerials. These devices may or may not require electrical contacts,depending on the type of nanowire material, such as magnetic nanowires.

For example, as described elsewhere herein, a thin film ofsemiconducting nanowires can be used in an electrical device, such assemiconductor device 200 shown in FIG. 2. Semiconductor device 200 is athree-terminal transistor device, having a source electrode 202, gateelectrode 204, and drain electrode 206. As shown in FIG. 2, thin film ofnanowires 100 couples source electrode 202 to drain electrode 206. Gateelectrode 204 is formed adjacent to thin film of nanowires 100. Duringoperation, when a sufficient voltage is applied to gate electrode 204,current can conduct between drain electrode 206 and source electrode202. In an alternative embodiment, gate electrode 204 need not bepresent, and device 200 operates as a two-terminal device, such as adiode. For example, in such an alternative embodiment, source electrode202 and drain electrode 206 can be cathode and anode electrodes.

In an embodiment, when thin film of nanowires 100 is formed from one ofmagnetic, ferroelectric, thermoelectric, piezoelectric, metallic, ortransition metal oxide materials, characteristics of the particularnanowire material can be exhibited during operation of the electricaldevice in which the thin film is incorporated.

For example, in an embodiment, when a current is applied to a thin filmof ferroelectric nanowires, an electric polarization can be caused inthe thin film of ferroelectric nanowires, of a permanent ornon-permanent type.

In another example embodiment, when a current is applied to a thin filmof piezoelectric nanowires, a stress can be generated in the thin filmof piezoelectric nanowires, which can be exhibited as change of shape, amovement, and/or a vibration of the thin film of piezoelectricnanowires. Such a thin film of piezoelectric nanowires could haveapplications in audio and other technology areas, for example.Transition metal oxide materials are example materials that can be usedto fabricate piezoelectric nanowires.

In another example embodiment, when current is applied to a thin film ofthermoelectric nanowires, heat can be transferred across the thin filmof thermoelectric nanowires. Such a thin film of thermoelectricnanowires can have many applications, such as in areas of temperaturecontrol and heating, and in particular, in applications where spatiallylocalized heating and/or cooling is desired.

Magnetic nanowires may not need to be coupled to electrical contacts inorder to operate. A thin film of magnetic nanowires can be formed on asurface to impart magnetic properties to the surface. The thin film ofnanowires can be sized as required by the particular application.

Metallic nanowires can be formed in a thin film in applications needingan efficient conductor. Furthermore, electrical devices such asinductors, transformers, and electromagnets can be formed from metallicnanowire thin films.

FIG. 7 shows a flowchart 700 providing example steps for making anelectrical device that incorporates a thin film of nanowires of thepresent invention, according to an embodiment of the present invention.Other structural embodiments will be apparent to persons skilled in therelevant art(s) based on the following discussion. These steps aredescribed in detail below.

Flowchart 700 begins with step 702. In step 702, a plurality of{magnetic; ferroelectric; thermoelectric; piezoelectric; metallic; ortransition metal oxide} nanowires are deposited onto a substrate. Forexample, the substrate can be substrate 208 shown in FIG. 2.Furthermore, the plurality of nanowires can be thin film of nanowires100, described above, wherein the nanowires of the thin film are madefrom magnetic, ferroelectric, thermoelectric, piezoelectric,metal(s)/alloys, or transition metal oxide materials. The nanowires canbe deposited onto the substrate such that the nanowires are randomlyaligned with respect to each other, or in alignment so that their longaxes are substantially parallel.

In step 704, first and second electrical contacts are formed on thesubstrate. For example, the first and second electrical contacts can besource electrode 202 and drain electrode 206, as shown in FIG. 2. Insuch an embodiment, a gate electrode can also be formed on thesubstrate. Alternatively, the first and second electrical contacts canbe cathode and anode electrodes. In further embodiments, the first andsecond electrical contacts can be other contact types. Furthermore,other numbers of contacts/additional contacts can be formed on thesubstrate.

Steps 702 and 704 can be performed in either order. An effect of steps702 and 704 is that at least one of the nanowires couples the firstelectrical contact to the second electrical contact. Subsequentprocessing steps can be performed as needed, such as those describedelsewhere herein, including patterning of the nanowires, doping of thenanowires, and causing/allowing the nanowires to adhere/attach to thesubstrate in contact with the electrical contacts.

Large-Area Macroelectronic Materials that Conduct Both Electrons andHoles with High Mobilities

While it is possible to incorporate both p- and n-dopants into a singlesemiconductor, such as silicon, the presence of both carrier types inthe same material leads to recombination and annihilation of thecarriers, resulting in poor mobility for either. Thus, a combination ofp-dopants and n-dopants in a single semiconductor by conventional meansis not desirable.

According to embodiments of the present invention, p-doped nanowires andn-doped nanowires can be separately fabricated, and deposited in ahomogeneous mixture onto a surface, such as a macroelectronic substrate.On a macroscopic level, the resulting material appears to contain a highconcentration of both n- and p-dopants. However, the individual carriertypes are physically separated from each other (i.e. they are indifferent nanowires), and as a result, they do not substantiallyinteract (i.e., the carriers do not substantially annihilate eachother). Thus, by creating such a mixture of p- and n-doped nanowires,macroelectronic devices can be fabricated that respond as if they areboth n- and p-doped. For example, a resulting thin film of nanowiresthat includes both n-doped and p-doped nanowires can exhibitcharacteristics of both n-doped and p-doped nanowires.

For example, diode, transistor, and other electrical devices describedelsewhere herein or otherwise known can be fabricated to include acombination of p-doped nanowires and n-doped nanowires. For instance,semiconductor device 200 shown in FIG. 2 can include a thin film ofnanowires 100 that includes a combination of p-doped nanowires andn-doped nanowires. The n-doped nanowires and p-doped nanowires can becombined in the thin film in a variety of ways. Some of these ways aredescribed as follows.

For example, FIG. 8A shows a close up view of an example portion of ananowire thin film 800 that includes a homogeneous mixture of n-dopednanowires 802 and p-doped nanowires 804. Thin film 800 can be used in avariety of electrical device types. Note that in the example of FIG. 8A,n-doped nanowires 802 and p-doped nanowires 804 can be formed and mixedprior to deposition onto a substrate, or can be mixed on a substrate.Also, while the nanowires in FIG. 8A are shown to be randomly oriented,the present invention includes the use of partially and substantiallyparallel oriented nanowires, as described above.

In another example embodiment, FIG. 8B shows an example portion of ananowire thin film 810 that includes both n-doped nanowires 802 andp-doped nanowires 804. For example, thin film 810 can be formed on asubstrate. As shown in FIG. 8B, a first region 812 of thin film 810includes a plurality of n-doped nanowires 802, and a second region 814of thin film 810 includes a plurality of p-doped nanowires 804. In theexample of FIG. 8B, first region 812 and second region 814 aresubstantially non-overlapping. Thus, as shown in the example of FIG. 8B,n-doped nanowires 802 and p-doped nanowires 804 can be deposited on asubstrate in a spatially or regionally segregated manner, eithersubstantially or even entirely segregated. The n-doped and p-dopednanowires can be spatially or regionally segregated in anyconfiguration, including forming segregated stripes, spots, etc. of then-doped and p-doped nanowires on the substrate.

In another example embodiment, FIG. 8C shows a thin film 820 ofnanowires that includes both n-doped nanowires 802 and p-doped nanowires804. As shown in FIG. 8C, a first sublayer 822 of thin film 820 includesa plurality of n-doped nanowires 802, and a second sublayer 824 of thinfilm 820 includes a plurality of p-doped nanowires 804. Thus, as shownin the example, of FIG. 8C, n-doped nanowires 802 and p-doped nanowires804 can be deposited on a substrate in two or more segregated layers.

In embodiments of the present invention, n-doped and p-doped nanowires802 and 804 can be mixed, regionally segregated within a layer, and/orsegregated into separate layers, in any combination.

FIG. 9 shows a flowchart 900 providing example steps for making a thinfilm of nanowires of the present invention, according to an embodimentof the present invention. The steps of FIG. 9 do not necessarily have tooccur in the order shown, as will be apparent to persons skilled in therelevant art(s) based on the teachings herein. Other structuralembodiments will be apparent to persons skilled in the relevant art(s)based on the following discussion. These steps are described in detailbelow.

Flowchart 900 begins with step 902. In step 902, a first plurality ofnanowires that are p-doped are formed. For example, the first pluralityof nanowires are p-doped nanowires 804, as shown in FIGS. 8A-8C. Thep-doped nanowires can be formed in any number of ways, such as thosedescribed elsewhere herein or otherwise known. For example, the p-dopednanowires may be grown as intrinsic nanowires that are subsequentlyp-doped, or can be grown from a p-doped semiconductor material.Furthermore, the p-doped nanowires can all be made from the samesemiconductor material (e.g., all p-doped Si), or there can be nanowiresmade different semiconductor materials (i.e., some p-doped Si nanowiresand p-doped CdS nanowires).

In step 904, a second plurality of nanowires that are n-doped areformed. For example, the second plurality of nanowires are n-dopednanowires 802, as shown in FIGS. 8A-8C. The n-doped nanowires can beformed in any number of ways, such as those described elsewhere hereinor otherwise known. For example, the n-doped nanowires may be grown asintrinsic nanowires that are subsequently n-doped, or can be grown froman n-doped semiconductor material. Furthermore, the n-doped nanowirescan all be made from the same semiconductor material (e.g., all n-dopedSi), or there can be nanowires made different semiconductor materials(i.e., some n-doped Si nanowires and n-doped CdS nanowires).

In step 906, the first plurality of nanowires and second plurality ofnanowires are deposited onto a substrate to form a thin film ofnanowires that includes n-doped and p-doped nanowires. For example, inembodiments, the first and second pluralities of nanowires can bedeposited separately or simultaneously. The first and second pluralitiesof nanowires can be mixed to form a single- or multi-layer homogeneousmixture. Alternatively, the first and second pluralities of nanowirescan be deposited onto two or more separate regions of the substrate,and/or into two or more separate sublayers of the thin film.

In an embodiment, where n-doped nanowires and p-doped nanowires are inseparate layers, flowchart 900 can include the step where another layerof material is formed between the layer of n-doped nanowires and layerof p-doped nanowires. For example, the middle layer can be an insulatorlayer, such as a plastic layer, a glass layer, an air layer, or otherinsulator layer type.

Thus, semiconductor devices/materials can be formed having both p-dopedand n-doped characteristics. In an embodiment, such devices/materialscan be viewed as having both n- and p-characteristics over a lengthscale greater than a factor X, where X is dependent on the size of thenanowires, the density of the nanowires, and/or other factors. Prior tothe present invention, such devices/materials did not exist.

Macroelectronic Devices Fabricated with Nanowire Heterostructures

In another embodiment of the present invention, electrical devices suchas p-n diodes, transistors, and other electrical device types, can befabricated using nanowire heterostructures. Nanowire heterostructures,as described herein, are nanowires that typically include a plurality ofp-n junctions along the length of the nanowire. In other words, nanowireheterostructures include alternating portions or segments along theirlengths that are different. For example, nanowire heterostructures canhave alternating portions that are differently doped and/or are madefrom different materials.

By forming nanowires that have differently portions, fabrication ofsemiconductor devices can be greatly simplified. For instance, in anembodiments where the portions are differently doped, because thenanowires are doped, regions of the substrate on which the nanowireswill be attached will not need to be doped using expensivephotolithography or ion implantation processes, as is conventionallydone. Furthermore, ohmic contact with a substrate is improved throughthe use of doped nanowires, as compared to intrinsic nanowires. Stillfurther, nanowire heterostructures do not need to be carefullypositioned on a substrate, as compared to conventional nanowires, asthey are typically much longer than a distance between electrodes on thesubstrate. Thus more of them will span the distance between theelectrodes, and therefore make contact with the electrodes, relative toconventional nanowires.

In embodiments where portions have different materials, manyapplications are possible. For example, a nanowire heterostructure couldinclude a first heavily p-doped silicon portion, a second heavilyn-doped GaN portion, and a third heavily n-doped silicon portion. Thiscould be used to simplify electrical interfacing to a GaN LED. Any othercombination of doping and materials in nanowire heterostructures can beused.

In a first embodiment, nanowire heterostructures are grown to havemultiple p-n junctions along their lengths. (Note that the followingdiscussion can be applied to other differences along the lengths of thenanowires, including different materials, etc. For illustrativepurposes, the discussion focuses on differently doped portions). Forexample, FIG. 10 shows a nanowire 1000 that is a nanowireheterostructure, according to an example embodiment of the presentinvention. As shown in FIG. 10, nanowire 1000 has a plurality of n-dopedportions 1010 a,b and a plurality of p-doped portions 1020 a,b. Thus, aplurality of p-n junctions 1030 a,b,c are present at intersections ofp-doped portions 1010 and n-doped portions 1020. Furthermore, each dopedportion of nanowire 1000 has a length 1002.

In embodiments, the lengths of doped portions of nanowire 1000 may ormay not be uniform. Preferably, the doped portions of a nanowireheterostructure of the present invention have lengths (i.e., in otherwords, the p-n junctions are spaced apart) approximately equal to thedistance between the electrodes in the final device in which thenanowire heterostructure will be used. For example, in an embodiment,the lengths of the doped portions can be equal to the distance betweenthe electrodes, or slightly longer than the distance between theelectrodes. In this manner, when the nanowire heterostructures aredeposited onto a substrate, on average, a single p-n junction of eachnanowire heterostructure will be located between the electrodes of thefinal device, and each nanowire heterostructure will be capable ofspanning the distance between the electrodes. Furthermore, due to thenumber of doped portions in each nanowire, the overall length of eachnanowire heterostructure will tend to be much greater than the distancebetween the electrodes. These attributes will create an efficient p-ndiode between each electrode pair, creating a macroelectronic device.Furthermore, manufacturing constraints are reduced by using nanowireheterostructures, as the nanowire heterostructures can be deposited ontoa substrate with little precision required, and can even be depositedrandomly. The repeating structure of the nanowire heterostructurestatistically allows for enough p-n junctions of randomly depositednanowire heterostructures to be located between electrical contacts tocreate operational devices. This manufacturing advantage of nanowireheterostructures is further illustrated below.

Using nanowire heterostructures, macroscopic heterostructured electronicdevices may be created. For example, FIG. 11A shows an exampletwo-terminal electrical device 1100 that includes a plurality ofnanowires 1000, according to an embodiment of the present invention. Asshown in FIG. 11A, nanowires 1000 a-e are deposited on a firstelectrical contact 1102 and a second electrical contact 1104, in anon-aligned fashion. For example, device 1100 can be a diode, and firstand second electrical contacts 1102 and 1104 can be cathode and anodeelectrodes. As shown in the embodiment of FIG. 11A, a distance betweenfirst electrical contact 1102 and second electrical contact 1104 isapproximately equal to length 1002 (e.g., either center-to-center orbetween inner edges), which is the length of the doped portions ofnanowires 1000 a-e. Furthermore, a total length of each of nanowires1000 a-e is greater than the distance between first electrical contact1102 and second electrical contact 1104. As shown in FIG. 11A, nanowires1000 c, 1000 d, and 1000 e do not couple together first and secondelectrical contacts 1102 and 1104. However, due to their locations,nanowires 1000 a and 1000 b do couple together first and secondelectrical contacts 1102 and 1104. Furthermore, nanowires 1000 a and1000 b have respective p-n junctions 1030 a and 1030 b that are locatedbetween first and second electrical contacts 1102 and 1104. Thus, device1100 can operate as a diode as configured in FIG. 11A.

Note that in the example of FIG. 11A, the p-n junctions 1030 a and 1030b of nanowires 1000 a and 1000 b, respectively, are directed oppositeeach other. In other words, the n-doped portion of nanowire 1000 a andthe p-doped portion of nanowire 1000 b are in contact with firstelectrical contact 1102, and the p-doped portion of nanowire 1000 a andthe n-doped portion of nanowire 1000 b are in contact with secondelectrical contact 1104. Thus, the p-n junctions 1030 a and 1030 b ofnanowires 1000 a and 1000 b, respectively, are conductive andnon-conductive in directions opposite to each other. Such a placement ofnanowires 1000 a and 1000 b can occur during a random deposition ofnanowire heterostructures on a substrate, for example. However, incertain applications, first and second electrical contacts 1102 and 1104will have voltages applied that will only allow for current to flow inone direction, causing only one of p-n junctions 1030 a and 1030 b ofnanowires 1000 a and 1000 b, respectively, to operate. For example,nanowires 1000 a and 1000 b may be light emitting nanowires (as furtherdescribed in the following subsection). Thus, when a sufficiently highvoltage is applied to first electrical contact 1102 relative to secondelectrical contact 1104, p-n junction 1030 b of nanowire 1000 b willconduct and emit light. When the high voltage is removed from firstelectrical contact 1102, no light is emitted by either of nanowires 1000a and 1000 b. Thus, in such an application, having the extra nanowireheterostructure(s) oriented in a direction opposite to that of currentflow will not affect operation. This embodiment creates a novel type oflight emitting diode, however, that emits light even when reversedbiased (i.e. when p-n junction 1030 a is forward biased and p-n junction1030 b does not emit light). However, in other applications, it may notbe desirable to have oppositely directed p-n junctions of nanowireheterostructures. Thus, in such applications, it may be desired todeposit nanowires in a fashion such that oppositely directed p-njunctions do not occur.

For example, in another embodiment, nanowires 1000 can be alignedrelative to each other and to the intended electrode pattern so thattheir p-n junctions 1000 are aligned. For example, this can accomplishedby chemically patterning the substrate such that nanowires 1000 onlystick or attach to defined locations. Furthermore, this can beaccomplished in other ways.

In a similar fashion to the creation of a two-terminal device, morecomplex devices, such as a p-n-p or n-p-n bipolar transistors, can beformed by creating nanowire heterostructures having p-n-p repeatingsegments spaced with a period equal to the distance between source anddrain electrodes. In an embodiment, for devices having more than oneinterface per device, it is desirable that, while the nanowireheterostructures have a built-in period substantially equal to thedistance between electrodes, the total length of the device within eachperiod be small relative to the total span. This will aid in decreasingthe number of nanowires that will bridge the gap between electrodes withonly a half a p-n-p segment at each end, rather than an entire p-n-psegment in the middle.

For example, FIG. 11B shows an example p-n-p transistor 1150,incorporating nanowire heterostructures, and having a drain electrode1152, a gate 1154, and a source electrode 1156 spaced at distances equalto length 1002 (e.g., typically the distance is center-to-center foradjacent contacts/electrodes). As shown in FIG. 11B, p-n junctions arepresent in each of nanowires 1000 a-c between drain and gate electrodes1152 and 1154, and between gate and source electrodes 1154 and 1156. Inan embodiment, for example, gate electrode 1154 can be separated fromnanowires 1000 a-c by a dielectric/insulating layer (not shown in FIG.11B). Thus, through deposition of nanowire heterostructures onelectrodes 1152, 1154, and 1156, and p-n-p transistor 1150 can beformed.

Using nanowire heterostructures, it is possible to form virtually anytype of electronic device from any material. For example, devices can bemade using nanowire heterostructures formed from any material, includingelectronic, semiconducting, optical, electro-optical, piezoelectric,thermoelectric, ferroelectric, and other materials.

As described above, nanowire heterostructures can be grown.Alternatively, photolithography, ion-implantation, or other dopingprocesses can be used to create the differently doped regions of thenanowire heterostructures such that macroscopic p-n junctions areformed. In an embodiment, this process can be done as a part of thepatterning process along with creating the electrodes.

Note that in embodiments, nanowire heterostructures can have more thantwo different dopant types. For example, nanowires 1000 have tworepeating dopant type portions: n-doped and p-doped portions (i.e., arepeating n:p doped pattern). Furthermore, nanowire heterostructures canother numbers of repeating dopant type portions, including p:n:p, n:p:n,p:i:p, n:i:n, p+:p:n, n+:n:p, p+:n:p, n+:p:n, and any other repeatingpattern.

FIG. 12 shows a flowchart 1200 providing example steps for making anelectrical device incorporating nanowire heterostructures, according toan embodiment of the present invention. The steps of FIG. 12 do notnecessarily have to occur in the order shown, as will be apparent topersons skilled in the relevant art(s) based on the teachings herein.Other structural embodiments will be apparent to persons skilled in therelevant art(s) based on the following discussion. These steps aredescribed in detail below.

Flowchart 1200 begins with step 1202. In step 1202, a plurality ofnanowires are formed so that each nanowire has along its long axis atleast one first portion doped with a first dopant and at least onesecond portion doped with a second dopant, each nanowire having aspacing between consecutive junctions of the first and second portionssubstantially equal to a first distance. For example, the plurality ofnanowires can be formed similarly to nanowire 1000 of FIG. 10. As shownin FIG. 10, nanowire 1000 is formed to have p-doped portions 1010 andn-doped portions 1020, which are respectively doped with n- and p-typedopant materials. Furthermore, as shown in FIG. 10, nanowire 1000 has aspacing between p-n junctions 1030 of a length 1002. A nanowireheterostructure can be formed to any length, having any number ofalternating doped portions.

In step 1204, a pair of electrical contacts are formed on the substrate,wherein a distance between the electrical contacts is approximatelyequal to the first distance. For example, in an embodiment, theelectrical contacts are first and second electrical contacts 1102 and1104, shown in FIG. 11A. As shown in FIG. 11A, the distance betweenfirst and second electrical contacts 1102 and 1104 is approximatelyequal to length 1002. Alternately, in an embodiment, the pair ofelectrical contacts are drain and gate electrodes 1152 and 1154 shown inFIG. 11B.

In step 1206, the plurality of nanowires are deposited onto thesubstrate, wherein at least one nanowire of the plurality of nanowirescouples the first electrical contact to the second electrical contact.For example, as shown in FIG. 11A, a plurality of nanowires 1000 a-e aredeposited onto the substrate. As shown in FIG. 11B, a plurality ofnanowires 1000 a-c are deposited onto the substrate. The plurality ofnanowires may be deposited onto the substrate in any fashion describedherein, or otherwise known. The plurality of nanowires may then becomeattached to the electrical contacts, according to any manner ofattachment.

Light Emitting Thin Films Using Nanowire Macroelectronics

In embodiments, nanowires fabricated from fluorescent semiconductormaterials, phosphorescent, electroluminescent, cathodoluminescent orother light-emitting materials, can be deposited onto a glass, plastic,or other substrate type to allow for low-cost, large-area lighting,emanating white or any other color of light. For example, an alignedmonolayer of semiconducting nanowires can be formed on a substrate forthis purpose. These nanowires can be fabricated from fluorescentmaterials, such as CdSe, GaN, InP or any other traditional ornon-traditional electroluminescent semiconductor material.

However, in order to form a light emitting diode (LED), a p-n junctionmust be present in each nanowire, between each electrode pair. This canbe difficult and costly to manufacture if each nanowire must bemacroscopically doped using post-fabrication ion-implantation, forexample.

Thus, according to the present invention, nanowires can be formed/grownwith p-n junctions in them, in a similar fashion as described above fornanowire heterostructures. As described above, each nanowire can beformed to have one or more p-n junctions. The p-n junctions arepreferably spaced apart at approximately the same distance as thedistance between the electrodes to which they will be attached in thefinal display/illumination device. Thus, as described above, when aquantity of nanowire heterostructures are deposited onto the substrate,there will be a distribution of p-n junctions located between theelectrodes, but no single p- or n-doped region will span the entiredistance between the electrodes. On average, every wire will span theentire gap or distance between the electrodes and will have one p-njunction between the electrodes. This creates an efficient LED in eachnanowire in which the p-n orientation is correct for the bias directionfor a macroelectronic LED. By patterning an array of anode and cathodeelectrodes, or source/drain electrodes, this can be done over anextremely large area, on a flexible substrate, if desired. Note that aunique aspect of the present invention is that if nanowires aredeposited such that the locations of the ends are random, roughly halfof the nanowires on a particular surface will have p-n junctions in theproper orientation and will therefore produce light when a bias isapplied in a particular direction, while the other half will not. Whenbiased in the opposite direction, the roles will be reversed for thedifferent populations of nanowires such that the second half of thenanowires will emit light while the first will not. As such, thistechnology enables a bi-polar LED. Through use of partial end-alignmentas described above, a more traditional uni-polar LED can also befabricated using the present invention.

A color or wavelength of light emitted by a nanowire depends, at leastin part, on the type of material from which the nanowire is fabricated,and upon a diameter of the nanowire. In an embodiment, by using a singlenanowire material and nanowire diameter for a plurality of nanowiresused in a display/illumination device, a monochromatic light source canbe created. In another embodiment, by using a mixture of nanowiresfabricated from different materials and/or having different diameters, apolychromatic light source can be created. For example, if red, blue,and green nanowires are included in the mixture, a white light sourcecan be created.

Light emitting nanowire thin films of the present invention allow forhigher quality color purity of light emissions, and for an improvedcolor index over conventional light emitters. These factors for lightemitting nanowire thin films are much higher than for OLED-basedversions of thin-film light emitters due to an ability to blend manypure colors, in embodiments of the present invention.

Quantum confinement effects in nanowires can be taken advantage of tocontrol the emission wavelength of an electroluminescent nanowire as afunction of diameter, while retaining the same material composition.This may be used to simplify the electrical interfacing to such a mixedfilm, because the composition of each light emitting element within themixed film is the same.

Note that the electrodes for the light emitting nanowiresheterostructures can be arranged in any pattern, as is desired for adisplay or illumination pattern. For example, FIG. 13A shows a discretepixel or light source 1300, having a pair of electrodes, firstelectrical contact 1302 and second electrical contact 1304, according toan embodiment of the present invention. First and second electricalcontacts 1302 and 1304 can be anode and cathode electrodes for an LED,or can be a pair of electrodes of a light emitting transistor. Aplurality of light emitting nanowires 1310 a-e are also present in lightsource 1300. Light emitting nanowires 1310 a-e, which are dopedsimilarly to nanowire 1000 of FIG. 10, and are fabricated from lightemitting materials, are coupled between first and second electricalcontacts 1302 and 1304. The p-n junction 1320 of each of light emittingnanowires 1310 a-e emits light when a sufficient voltage is applied tofirst electrical contact 1302 relative to second electrical contact1304. Although nanowires 1310 a-e are shown to be aligned in FIG. 13A,in alternative embodiments, nanowires 1310 do not need to be aligned,and can be randomly distributed, for example.

In another example embodiment, FIG. 13B shows a column of discretepixels or light sources 1300 a-c, each similar to pixel or light source1300. In a similar fashion, any number of pixels or light sources 1300can be arranged in rows, columns, or in a two-dimensional array, to beused in a display having a large number/plurality of pixels, forexample. Note that, as shown in FIG. 13B, light sources 1300 a-c canhave a common second electrical contact 1304, if desired, and can stillbe independently controlled by first electrical contacts 1302 a-c,respectively. Alternatively, independent second electrical contact arealso possible.

In another example embodiment, FIG. 13C shows a large area light source1320, which includes a plurality of light source columns 1350, accordingto an embodiment of the present invention. Light source 1320 includeselongated first and second electrical contacts 1324 and 1326, which arearranged to illuminate p-n junctions in multiple columns 1350 a-c (orrows) to provide illumination over a large area. First and secondelectrical contacts 1324 and 1326 can have any lengths, can have anynumber of bends and/or trace “fingers”, and can interlock in any numberof columns and/or rows to allow for nanowires to bridge them, to providelight over a relatively large area (and/or provide other diodefunction).

Note that the spacing between electrodes can be selected for optimumelectrical performance. The length of electrodes can be arbitrarilylong, to maximize a total fluorescent/illumination output and to reducea number of required processing steps.

In another embodiment, nanowires 1310 can be aligned relative to eachother and to the intended electrode pattern so that their p-n junctions1320 are aligned. For example, this can accomplished by chemicallypatterning the substrate such that nanowires 1310 only stick or attachto defined locations. This can create a more efficient light-emittingdevice, and cause less statistical variation between formed light sourcedevices. Alternatively, nanowires 1310 can be deposited randomly orisotropically oriented. In such an embodiment, a relatively lessefficient light-emitting device may be created (e.g., fewer of nanowires1310 may be coupled to the electrodes, and thus not operational), andcan cause relatively more statistical variation between formed lightsource devices.

Furthermore, a light source can include nanowires 1310 formed in a filmthat is a mono-layer of nanowires 1310 (i.e., one nanowire thick layer),a sub-monolayer of nanowires 1310, or multiple layers of nanowires 1310.

In addition, it is possible to create a light-emitting film without theneed for fabricating p-n junctions within the nanowires. This can beaccomplished through post deposition lithography and implantation, orthrough the fabrication of a Schotky diode at thesemiconductor-electrode interface.

FIG. 14 shows a flowchart 1400 providing example steps for making alight emitting device incorporating light emitting nanowireheterostructures, according to an embodiment of the present invention.The steps of FIG. 14 do not necessarily have to occur in the ordershown, as will be apparent to persons skilled in the relevant art(s)based on the teachings herein. Other structural embodiments will beapparent to persons skilled in the relevant art(s) based on thefollowing discussion. These steps are described in detail below.

Flowchart 1400 begins with step 1402. In step 1402, at least one lightemitting semiconductor material is selected. For example, thesemiconductor material can include one or more fluorescent materials, orother light emitting materials, such as CdSe or InP, that are suitablefor light emitting nanowire applications. A single material can beselected for a monochromatic light source, or multiple materials can beselected to fabricate different nanowires, so that a polychromatic lightsource can be created.

Note that some materials are light emitting at a nanoscale size, eventhough the material does emit light when at a larger size. Thesematerials are suitable for use in light emitting nanowires. For example,some bulk semiconductor materials are not light emitting, while anano-sized particle of the same material is. This includes silicon,which is typically not a light-emitting semiconductor, but does begin toemit light below a certain critical size. Any of these light emittingemiconductor material types are applicable to the present invention.

Note that flowchart 1400 can optionally include the step of selectingdiameter(s) for the nanowires.

In step 1404, a plurality of nanowires are formed from the selected atleast one light emitting semiconductor material. For example, theplurality of nanowires are nanowires 1310 a-e, shown in FIG. 13A. Notethat any number of light emitting nanowires can be created for aparticular operation, including numbers in the 10s, 100s, 1000s,millions, and other numbers. In an embodiment, note that step 1404 caninclude the step of forming one or more of the plurality of nanowires tohave the selected diameter(s).

In step 1406, each nanowire is doped so that each nanowire includes atleast one p-n junction. In an embodiment, each nanowire is doped to havea plurality of alternating n- and p-doped portions. For example, thenanowires are doped in a similar fashion as shown for nanowire 1000shown in FIG. 10, and as shown for nanowires 1310 a-e shown in FIG. 13A.The nanowires can be doped to have any number of alternating n- andp-doped portions.

Note that in an embodiment, steps 1404 and 1406 are performedconcurrently, where the n- and n-doped regions are formed in thenanowires as the nanowires are grown. Alternatively, steps 1404 and 1406can be performed separately. Furthermore, in an embodiment, eachnanowire is formed to have a distance between consecutive junctions ofalternately doped portions substantially equal to a first distance.

In step 1408, the plurality of nanowires are deposited onto a substrate.For example, the plurality of nanowires are deposited on a substrate asshown in FIGS. 13A-13C. The nanowires can be deposited in any manner,and can be deposited onto any substrate type described elsewhere herein,or otherwise known. The substrate can be any size, including small orlarge, such as a large area macroelectronic substrate. In an embodiment,the substrate has first and second electrical contacts formed thereon.In an embodiment, at least one nanowire of the plurality of nanowirescouples the first electrical contact to the second electrical contact.Furthermore, the electrical contacts can be spaced apart at the distancebetween p-n junctions in the nanowires.

Note that in an alternative embodiment for a light emittingsemiconductor device, nanowires are deposited onto a substrate, couplinga first electrical contact to a second electrical contact. At least oneof the first and second electrical contacts is a metal contact. Duringoperation of the light emitting device, light can be caused to beemitted from a junction of the nanowires and the metal electricalcontact, due to the Schottky effect, in a similar fashion to the way aSchottky diode operates. In such an embodiment, the nanowires used inthe device do not necessarily have to be nanowire heterostructrures, andcan instead be uniformly doped/undoped nanowires, if desired.

Nanowire Solution Flow Mask Embodiments

As described above, many electronic and other type devices canincorporate nanowires. In this sub-section, techniques are desired foraccurate positioning of nanowires.

In a large-scale manufacturing context, nanowire placement techniquesare desired for devices manufactured in large quantities. For example,for each device manufactured, the nanowires must be positioned onsurfaces of the device to make electrical contacts as needed. However,the devices must be manufactured in a commercially/economicallypractical manner. The nanowires must be accurately positioned, andplaced on the surface(s) in sufficient quantity/density. In some cases,the nanowires must be placed in alignment with each other. Thus,techniques are desired for placement of nanowires according to thesemanufacturing requirements, in a commercially/economically practicalmanner.

Furthermore, in a quality control context, after nanowires aremanufactured, it may be desirable to test some of the manufacturednanowires. For example, it may be desirable to test whether thenanowires are sufficiently conductive, to measure an impedance of thenanowires, and/or to measure other electrical/mechanical attributes ofthe nanowires. Thus, it may be desirable to position the nanowires in acommercially practical manner on a surface in contact with testelectrodes.

According to the present invention, nanowires in a solution are flowedover a surface. The nanowire solution is flowed over the surface in amanner such that one or more nanowires of the solution reside on oradhere to the surface. The nanowire solution flow is then removed, andthe one or more nanowires remain on the surface to form one or moreelectrical connections and/or other connection types.

In an embodiment, a flow mask is used to direct the flow of nanowiresonto a designated portion of a surface. The flow mask thus allows forthe positioning of the nanowires on the designated portion of thesurface. In an embodiment, using the flow mask, the nanowires arepositioned in alignment with each other. In other embodiments, thenanowires are not placed in alignment by the flow mask

In an embodiment, the present invention allows for nanowires to bepositioned with low probability of forming a parallel circuit. In otherwords, the present invention allows for deposition of nanowires at adesired density, so that electrical connections are formed by a singlenanowire, if desired. Furthermore, devices are provided that also enablerapid measurement of wire and contact resistances in both wet chemicaland dry environments.

This section describes example flow techniques and flow mask embodimentsfor positioning nanowires on surfaces. Nanowires can be positioned bythe flow mask on semiconductor wafers, electronic substrates, or anyother surface, for use in electronic devices, in electrical testconfigurations, and in any other devices or systems. The particularembodiments described herein are provided for illustrative purposes, andare not limiting. Alternative embodiments for flow masks will beapparent to persons skilled in the relevant art(s) from the teachingsherein. These alternative embodiments are within the scope and spirit ofthe present invention.

FIGS. 15A and 15B show bottom and cross-sectional views of an exampleflow mask 1500, according to an embodiment of the present invention. Asshown in FIGS. 15A and 15B, flow mask 1500 includes a body 1502. Body1502 includes an input port 1510 and an output port 1520. A firstsurface 1504 of body 1502 is configured to mate with a target surface.For example, the target surface may be a surface of a wafer, a surfaceof a substrate, such as a macro-electronic substrate, or surface of anyother structure. For instance, the target surface can be a surface of asemiconductor wafer that has an array of integrated circuits formedthereon. Flow mask 1500 can be applied to the semiconductor wafersurface to position nanowires on any or all of the integrated circuits.Flow mask 1500 can be sized to cover an entire wafer, or to cover anyportion thereof.

As shown in FIG. 15A, first surface 1504 has a plurality of channels1506 a-1506 e formed therein. First surface 1504 can have any number ofone or more channels 1506 formed therein. Channels 1506 can besubstantially parallel to each other, as shown in FIG. 15A, or they canbe formed in one or more different directions, as is desired fordeposition of nanowires. As shown in FIG. 15A, first surface 1504 alsohas first and second feeder channels 1530 a and 1530 b formed therein.First feeder channel 1530 a is an input feeder channel coupled betweeninput port 1510 and a first end of channels 1506. Second feeder channel1530 b is an output feeder channel coupled between a second end ofchannels 1506 and output port 1520. First and second feeder channels1530 a and 1530 b are optionally present. Furthermore, when present,first and second feeder channels 1530 a and 1530 b may be exposed onfirst surface 1504 (as shown in FIG. 15A) or may be internal to flowmask 1500. In embodiments where one or both of first and second feederchannels 1530 a and 1530 b are not present, each of channels 1506 may bedirectly coupled to input port 1510 and/or to output port 1520.Alternatively, an input port 1510 and/or an output port 1520 can bepresent for each channel.

As shown in FIGS. 15A and 15B, input port 1510 is coupled to a first endof channels 1506 a-e through first feeder channel 1530 a, and outputport 1520 is coupled to a second end of channels 1506 a-e through secondfeeder channel 1530 b. Input port 1510 is formed in body 1502 to supplya flow of nanowires to channel 1506 a-e. Output port 1520 is formed inbody 1502 to remove the flow of nanowires from channels 1506 a-e. Thus,a flow of nanowires is directed into input port 1510 of body 1502, andflows through first feeder channel 1530 a. First feeder channel 1530 adisperses the flow of nanowires to channels 1506 a-e. Second feederchannel 1530 b collects the flow of nanowires from channels 1506 a-e.The flow of nanowires flows from second feeder channel 1530 b to outputport 1520, where it is removed from flow mask 1500. When first surface1504 is mated with a target surface, channels 1506 a-e each cover aportion of the target surface. Channels 1506 a-e are formed in firstsurface 1504 to allow nanowires of the flow of nanowires to bepositioned on a portion of the target surface covered by channels 1506a-e.

The length, width, and/or depth of channels 1506 can be selected tocontrol the flow of nanowires, and to optimize/control the placement andorientation of nanowires on the target surface. These parameters can beoptimized for a particular length/width of nanowires of the flow ofnanowires. Furthermore, these parameters can be optimized for aparticular target conductive pattern on the target surface. For example,flow mask 1500 can have channels of any width, including widths in theones, tens, hundreds, and thousands of micrometers. For example, fornanowires of 15 μm length, channel widths can in be the range of 1-1000μm, including 100 μm, 500 μm, 700 μm, and other width channelstherebetween and otherwise. Furthermore, a channel 1506 can include aplurality of channel segments, separated by conduits (i.e., tubes ortunnels in flow mask 1500) that are internal to flow mask 1500. In thismanner, a channel 1506 can position nanowires on discrete, separateareas of the target surface, along the channel 1506.

Flow mask 1500 can be formed from various materials, including a metalor combination of metals/an alloy, a plastic, a polymer, glass, asubstrate material, and other material. Flow mask 1500 can be molded,machined, etched, and/or otherwise formed. Flow mask 1500 can be madeany size as required. For example, a four inch diameter or square flowmask can be used to interface with a four inch substrate or wafer.

A variety of systems can incorporate flow mask 1500 for positioningnanowires, according to embodiments of the present invention. Forexample, FIG. 16 shows a nanowire positioning system 1600 thatincorporates flow mask 1500, according to an example embodiment of thepresent invention. As shown in FIG. 16, system 1600 includes flow mask1500, a target surface 1602, a nanowire solution source 1604, and ananowire solution receptacle 1606. Furthermore, as shown in FIG. 16,nanowire solution source 1604 contains a nanowire solution 1650.Nanowire solution 1650 is typically a liquid solution containing aplurality of nanowires. The composition of nanowire solution 1650 can beselected to aid in transferring nanowires from the flow of nanowiresthrough flow mask 1500 to target surface 1602.

As shown in FIG. 16, first surface 1504 of flow mask 1500 is mated withtarget surface 1602. Flow mask 1500 is configured to mate with targetsurface 1602 so that a substantially leak-proof seal is formedtherebetween. Thus, nanowire solution 1650 will be substantiallycontained within an enclosure formed between flow mask 1500 and targetsurface 1602 when flowing therethrough. In an embodiment, first surface1504 of flow mask 1500 is substantially flat or planar (exclusive ofchannels 1506 and feeder channels 1530, when present) to mate with aflat or planar target surface 1602, to form a seal. However, inalternative embodiments, first surface 1504 can be otherwise contouredto mate with target surface 1602. Furthermore, flow mask 1500 and targetsurface 1602 can have interlocking tab(s) and slot(s) to allow them tomate/align properly with each other. Flow mask 1500 and target surface1602 can alternatively be optically and/or otherwise mechanicallymated/aligned. A sealing material may be used between flow mask 1500 andtarget surface 1602 to aid in maintaining a seal, although such asealing material is not required in all applications.

Nanowire solution source 1604 is coupled to input port 1510 of flow mask1500. Nanowire solution source 1604 supplies nanowire solution 1650 toinput port 1510 to supply a flow of nanowires through flow mask 1500,across target surface 1602. Nanowire solution receptacle 1606 is coupledto output port 1520 of flow mask 1500 to receive and remove the flow ofnanowires from flow mask 1500, and in embodiments, can store thereceived nanowire solution 1650. In an embodiment, nanowire solutionsource 1604 can supply a pressurized flow of nanowires through flow mask1500. Furthermore, in an embodiment, nanowire solution source 1604 canprecisely control a flow rate of nanowire solution 1650 through flowmask 1500.

FIGS. 17A and 17B show plan and cross-sectional views of a flow 1702 ofnanowires flowing through flow mask 1500, according to an exampleembodiment of the present invention. The direction/path of flow 1702through flow mask 1500 is indicated generally by arrows. Furthermore,additional arrows 1710 indicate a path that some nanowires of flow ofnanowires 1702 take to leave nanowire solution 1650 to become positionedon target surface 1602. In an embodiment, the nanowires becomepositioned on target surface 1602 in an orientation substantiallyparallel to a direction of flow 1702 through channels 1506.

FIG. 18A shows a plan view of an example semiconductor wafer 1800 matedwith flow mask 1500 (only channel positions of flow mask 1500 are shownin FIG. 18A). The positions of channels 1506 a-e of flow mask 1500relative to wafer 1800 are indicated by dotted lines. Each of channels1506 a-e are positioned to cover a corresponding one of portions 1802a-e of wafer 1800.

FIG. 18B shows nanowire regions 1810 a-e (i.e., nanowire depositionregions) on the surface of wafer 1800 having nanowires positionedthereon, due to operation of the present invention.

FIG. 18C shows further detail of an example, wafer 1800 showing alocation of an array of integrated circuits 1820 a-n formed thereon.Integrated circuits 1820 can be any integrated circuit type, and anysize, including 0.5 cm². As shown in FIG. 18C, a portion of eachintegrated circuit 1820 a-n is covered by a portion of one of nanowireregions 1810 a-e. Thus, nanowires are positioned by the flow mask of thepresent invention on portions of each of integrated circuits 1820 a-n.For example, the integrated circuits can each include a plurality ofelectrically conductive traces. The nanowires of nanowire regions 1810a-e form one or more connections between electrically conductive tracesof each integrated circuit 1820.

For example, FIG. 19A shows an integrated circuit 1900, which can be anexample of one of integrated circuits 1820. As indicated in FIG. 19A, aportion 1802 of integrated circuit 1900 is covered by a channel of aflow mask. FIG. 19B shows a close-up view a portion 1920 of integratedcircuit 1900, showing detail of example electrically conductive traces1902. FIG. 19C shows a view of portion 1920, after nanowires 1910 havebeen deposited thereon in nanowire region 1810 by operation of flow mask1500. As shown in FIG. 19C, after operation of flow mask 1500, nanowires1910 are positioned to form electrical connections between varioustraces of electrically conductive traces 1902. For example, as shown inFIG. 19C, a nanowire 1910 a forms a connection between a trace finger ofa ground signal trace 1904 and a trace finger 1956.

Note that the density of nanowires 1910 deposited on an integratedcircuit, or other surface, can be controlled in various ways, including:varying the rate of flow of nanowires through flow mask 1500; selectinga density of nanowires in nanowire solution 1650; controlling acomposition of nanowire solution 1650 (e.g., selecting the base solutiontype, etc.); selecting a length of time to apply the flow of nanowires,etc. The density of nanowires 1910 can thereby be controlled tostatistically control how many of nanowires 1910 will be allowed to makeeach connection on integrated circuit 1900. Furthermore, the spacingbetween electrodes, the thickness of electrodes, the width of channels1506, etc., can be used to control how many of nanowires 1910 will beallowed to make each connection.

Furthermore, note that nanowires 1910 can be deposited on the substrateby flow mask 1500 before or after the pattern of integrated circuit 1900is formed on the substrate. For example, if nanowires 1910 are depositedfirst, the traces of integrated circuit 1900 are then formed on thesubstrate, on nanowires 1910.

FIG. 20A shows a graph 2000 related to FIGS. 19A-C. Graph 2000 showsvarious probabilities related to forming electrical connections forintegrated circuit 1900, according to the present invention. The Y-axisof graph 2000 indicates a probability, and the X-axis of graph 2000indicates a number of nanowires per length (1/μm). The example of graph2000 relates nanowires of an example length of 15 μm being applied tointegrated circuit 1900 having trace separation of 12 μm in the regionof nanowire deposition. Line 2002 indicates a probability of an opencircuit. Line 2004 indicates a probability of a single nanowire forminga conductive path between conductive traces. Line 2006 indicates aprobability of more than a single nanowire forming a conductive pathbetween conductive traces. As shown on line 2004 in FIG. 20A, for atrace separation of 12 μm, and nanowires having an average length of 15μm, a maximum probability for a single nanowire forming a conductivepath between conductive traces is around 0.35, at a range of 0.3-0.4nanowires/length of the nanowire solution being applied, where:

$\;{\begin{matrix}{{nanowires}\text{/}{length}} \\\left( {1/{\Phi m}} \right)\end{matrix} = {\begin{matrix}{a\mspace{14mu}{density}\mspace{14mu}{of}\mspace{14mu}{nanowires}\mspace{14mu}{in}} \\{{the}\mspace{14mu}{nanowire}\mspace{14mu}{solution}\mspace{14mu}\left( {1\text{/}\Phi\mspace{14mu} m^{2}} \right)}\end{matrix} \times \begin{matrix}{{electrical}\mspace{14mu}{contact}} \\{{width}\mspace{14mu}\left( {\Phi\mspace{14mu} m} \right)}\end{matrix}}}$

FIG. 19D shows another example integrated circuit 1950, which can be anexample of integrated circuit 1820. As indicated in FIG. 19D, a portion1802 of integrated circuit 1950 is covered by a channel of a flow mask.FIG. 19E shows a close-up view of a portion 1960 of integrated circuit1950. FIG. 19F shows a view of portion 1960, after nanowires 1910 havebeen deposited thereon in nanowire region 1810 by operation of flow mask1500. As shown in FIG. 19F, after operation of flow mask 1500, nanowires1910 are positioned to form electrical connections between varioustraces of electrically conductive traces 1952 of integrated circuit1950. For example, a nanowire 1910 a forms a connection between tracefingers 1954 and 1956.

FIG. 20B shows a graph 2050 related to FIGS. 19D-19F that is similar tograph 2000. Line 2052 indicates a probability of a single nanowireforming a conductive path between conductive traces/electrodes. Line2054 indicates a probability of more than a single nanowire forming aconductive path between conductive traces. As shown on line 2052 in FIG.20B, for a trace separation of 4 μm, and nanowires having an averagelength of 15 μm, a maximum probability for a single nanowire forming aconductive path between conductive traces is around 0.06, at about 0.05nanowires/length of the nanowire solution being applied. By varyingelectrical contact spacing, nanowire length, and/or other parameters,the probabilities shown in FIGS. 20A and 20B can be altered, andoptimized for a particular application.

Note that the integrated circuits described above can be any type ofintegrated circuit device, for use in an end product, for example.Alternatively, these integrated circuits can be used for testing offabricated nanowires. For example, nanowires can be positioned on anintegrated circuit (or other circuit type) using the flow mask of thepresent invention to test whether the nanowires are sufficientlyconductive, to measure an impedance of the nanowires, and/or to measureother electrical/mechanical attributes of the nanowires. Two point andfour point test probe devices can be used to test the nanowires on theintegrated circuits, for example. For example, integrated circuit 1950of FIG. 19D can be used in a four-test probe system. Test probes can becoupled to two of the pads near the outer edges of integrated circuit1950 to test a nanowire coupling together traces that correspond to thetwo pads. A pair of probes that measure a resistance between the pads,or other parameter, due to the nanowire(s) can be coupled to the pads,while another pair of probes are used to supply a test current. In asimilar fashion, integrated circuit 1900 of FIG. 19A can be used in atwo-test probe system, where trace 1904 shown in FIG. 19C is coupled toground or other reference potential. Alternatively, integrated circuits1900 and 1950 can be used as electrical devices, where nanowires areflowed thereon to create electrical connections, diodes, transistors,etc.

FIG. 21 shows a flowchart 2100 providing example steps for positioningnanowires on a target surface using a flow mask, according to an exampleembodiment of the present invention. Other structural and operationalembodiments will be apparent to persons skilled in the relevant art(s)based on the following discussion. These steps are described in detailbelow.

Flowchart 2100 begins with step 2102. In step 2102, a flow mask is matedwith the target surface. For example, in an embodiment, the flow mask isflow mask 1500 shown in FIGS. 15A and 15B. As shown in FIG. 16, firstsurface 1504 of flow mask 1500 is mated with target surface 1602.

In step 2104, a liquid that contains a plurality of nanowires is flowedthrough at least one channel in a surface of the flow mask. For example,as shown in FIGS. 17A and 17B, the liquid containing a plurality ofnanowires is a nanowire solution, such as nanowire solution 1650. Asshown in FIG. 17A, nanowire solution 1650 flows through flow mask 1500in the direction of flow 1702. Flow 1702 of nanowires flows through oneor more channels, such as channels 1506 a-e shown in flow mask 1500 ofFIG. 15A. As shown in FIG. 18A, channels 1506 cover portions 1802 of anexample target surface.

In step 2106, nanowires contained in the liquid flowing through the atleast one channel are permitted to become positioned on the portion ofthe target surface covered by the at least one channel. For example, asindicated by arrows 1710 in FIG. 17B, nanowires are permitted to becomepositioned on target surface 1602 from flow 1702. In an embodiment, thenanowires positioned on target surface 1602 form at least one electricalconnection between electrically conductive features on target surface1602. In an embodiment, a density of nanowires in the nanowire solutioncan be selected so that a sufficient number of nanowires are permittedto become positioned on the covered portion of the target surface toform the electrical connections

In step 2108, the flow of the liquid through the at least one channel isdiscontinued. In this manner, nanowires are allowed to remain positionedon the portion of the target surface. For example, as shown in FIG. 18B,nanowires remain positioned on portions of target surface 1602,indicated as nanowire regions 1810.

In an embodiment, flowchart 2100 can include a further step, wherein thenanowires are caused to become attached (or adhere) to target surface1602. For example, the nanowire solution can be cured on target surface1602 to adhere the nanowires to target surface 1602. Alternatively, anepoxy or other adhesive material can be added to the nanowires on targetsurface 1602 to cause them to become attached.

Spraying of Nanowires on Substrates

As described above, techniques are desired for positioning nanowires onsubstrates, and in particular, on large area substrates. One reason forthis is that it is very difficult or impossible to apply conventionalphotolithographic techniques to large area substrates.

According to an embodiment of the present invention, spraying techniquesare used to attach nanowires to surfaces, including a large areasubstrate. The nanowires can be sprayed on the surfaces to formelectrical connections between electrodes, or for other reasons. Thenanowires can be sprayed onto the electrical contacts on the surface, orthe nanowires can be sprayed onto the surface first, and then theelectrical contacts can be metallized or otherwise formed directlythereon.

According to an embodiment of the present invention, the sprayingtechniques are used to direct a flow of nanowires onto a designatedportion of a surface. For example, a spray nozzle is used to spray ananowire solution onto a surface, to position the nanowires ondesignated portions of the surface. In an embodiment, using spraytechniques, the nanowires are positioned in alignment with each other.In other embodiments, spraying the nanowire solution does not align thenanowires.

This section describes spray techniques and example spray nozzleembodiments for placing and positioning nanowires on surfaces. Nanowirescan be positioned by the spray nozzle on semiconductor wafers,electronic substrates, or any other surface, for use in electronicdevices, in electrical test configurations, and in any other devices orsystems. The particular embodiments described herein are provided forillustrative purposes, and are not limiting. Alternative embodiments forthe spray nozzles/techniques will be apparent to persons skilled in therelevant art(s) from the teachings herein. These alternative embodimentsare within the scope and spirit of the present invention.

FIG. 22 shows a block diagram of an example nanowire spray applicationsystem 2200, according to an embodiment of the present invention. System2200 includes a nozzle 2202, a target surface 2204, a nanowire solutionsource 2206, and a nanowire solution conduit 2208. System 2200 canalternatively have other configurations, as would be understood bypersons skilled in the relevant art(s).

As shown in FIG. 22, nanowire solution source 2206 contains a nanowiresolution 2210. Nanowire solution 2210 is typically a liquid solutioncontaining a plurality of nanowires. The composition of nanowiresolution 2210 can be chosen to optimize the flow of nanowires throughnanowire solution conduit 2208 and/or nozzle 2202 onto target surface2204. For example, the viscosity of nanowire solution 2210 can beselected to enhance the flow of nanowires onto target surface 2204. Thecomposition of nanowire solution 2210 can be selected depending on thesize of the nanowires in nanowire solution 2210. Target surface 2204 maybe a surface of a wafer, a surface of a substrate, such as amacro-electronic substrate, or surface of any other structure, eitherdescribed elsewhere herein, or otherwise known.

Nanowire solution source 2206 provides nanowire solution 2210 to nozzle2202 through nanowire solution conduit 2208. Nanowire solution conduit2208 can be any type of conduit for a fluid or solution, includingtubing, pipes, and/or valves. Note that in some embodiments nanowiresolution conduit 2208 is not necessary, where nanowire solution source2206 is coupled directly to nozzle 2202.

Nozzle 2202 is coupled to nanowire solution source 2206, to receive aflow of nanowires. Nozzle 2202 directs and/or controls a flow ofnanowires onto target surface 2204. FIG. 23 shows a detailed view ofnozzle 2202 outputting a flow 2302 of nanowires onto an example targetsurface 2204. In FIG. 23, target surface 2204 is a substrate 2306 thatis supported by a substrate carrier 2308. Nozzle 2202 can have anynumber of one or more openings for providing the flow of nanowires ontotarget surface 2204. For example, nozzle 2202 can have a single opening.Alternatively, as shown in FIG. 23, nozzle 2202 can have a plurality ofopenings 2304 a-e. Openings 2304 can be arranged in a single row orcolumn of openings, can be arranged in a two-dimensional array ofopenings, or can be arranged in any other manner. Furthermore, openings2304 can have any shape, including round, elliptical, rectangular, orother shape. Openings 2304 can be of any width, including in widths thatare factors of 10's and 100's of microns. For example, a size ofopenings 2304 can be used to dictate a size of droplets of nanowiresolution 2210 being applied to target surface 2204.

A controlled pressurized, aerosol, or jet spray source can be used tocause the flow of nanowires to spray from nozzle 2202 at desired rate.The pressure required to spray nanowires from nozzle 2202 at aparticular rate can be determined on an application-by-applicationbasis.

FIG. 24 shows a plan view of target surface 2204 having a plurality ofnanowires 2402 positioned thereon, due to application of the presentinvention. Nanowires 2402 were deposited by nozzle 2202 from flow 2302of nanowires. As shown in FIG. 24, nanowires 2402 are positioned ontarget surface 2204 in a single, substantially uniform distribution ofnanowires. Nanowires 2402 can be positioned in a single distributionarea through the use of a single opening 2304 in nozzle 2202, or throughthe use of a plurality of openings 2304 in nozzle 2202 that haveadjacent or overlapping coverage areas on target surface 2204.Furthermore, nanowires 2402 are positioned on target surface 2204 inalignment (i.e., parallel to each other). In embodiments, nanowires 2402can be aligned with respect to each other through the use of nozzle2202. Nanowires 2402 can be aligned by nozzle 2202 depending on the sizeof openings 2304 in nozzle 2202, the viscosity of nanowire solution2210, the size of the nanowires 2402, and other factors mentionedelsewhere herein. For example, droplets of nanowire solution 2210applied to target surface 2204 can have a surface tension that alignsnanowires 2402.

FIG. 25 shows a plan view of target surface 2204 having a secondplurality of nanowires 2502 positioned thereon, due to application ofthe present invention. Nanowires 2502 were deposited by nozzle 2202 fromflow 2302 of nanowires. As shown in FIG. 25, nanowires 2502 arepositioned on target surface 2204 in a plurality of distribution areas2504 a-d. Nanowires 2502 can be positioned in a plurality ofdistribution areas 2504 a-d through the use of a single opening 2304 innozzle 2202, that is directed/moved to deposit nanowires in multiplenon-overlapping areas, or through the use of a plurality of openings2304 in nozzle 2202 that have non-adjacent or non-overlapping coverageareas on target surface 2204. Furthermore, nanowires 2502 are positionedon target surface 2204 in alignment (i.e., parallel to each other). Inembodiments, openings 2304 can be configured to cause nanowires to berandomly aligned (i.e., non-necessarily parallel to each other) ontarget surface 2204.

Electronic contacts can be formed on target surface 2204 before or afterdeposition of nanowires 2402 by nozzle 2202. FIG. 26 shows a plan viewof target surface 2204 of FIG. 24 with a plurality of electricalcontacts 2602 a-e formed thereon, in electrical contact with nanowires2402. Nanowires 2402 form electrical connections between adjacent pairsof electrical contacts 2602 a-e. Electrical contacts 2602 can be anyelectrical contact type. For example, adjacent pairs of electricalcontacts 2602 can be anode and cathode electrodes. Alternatively, threeadjacent electrical contacts 2602 can function as drain, gate, andsource electrodes of a transistor.

Any type of nanowire described herein or otherwise known can bedeposited using spray techniques of the present invention. The densityof nanowires in nanowire solution 2210 can be controlled to create adesired density of nanowires deposited on target surface 2204.Furthermore, once nanowires have been deposited on target surface 2204using spray techniques, the nanowires/nanowire solution can be cured orset on target surface 2204 in any manner described elsewhere herein orotherwise known. After nanowires have been deposited on target surface2204, any process for defining features in the nanowire film on targetsurface 2204 can be used, including lithographic and washing techniques,for example. Furthermore target surface 2204 can be prepared/treated toenhance adherence/attachment of nanowires in any manner describedelsewhere herein, or otherwise known.

A size/diameter of openings 2304 can be configured to enhance the flowof nanowires onto target surface 2204, such as causing nanowires to bedeposited in alignment. In an embodiment, a width of one or moreopenings 2304 can be made greater than or equal to (≧) a length of thenanowires to be spray deposited. Alternatively, a width of one or moreopenings can be less than a length of the nanowires. In an embodiment, awidth of an opening 2304 can be in a range of 1 μm to 1000 μm, althoughwidths outside of this range can also be used, depending upon theparticular application. Furthermore, openings 2304 can have any shape,including round, elliptical, rectangular, or other shape.

FIG. 27 shows a flowchart 2700 providing example steps for positioningnanowires on a target surface using spray techniques, according to anexample embodiment of the present invention. Other structural andoperational embodiments will be apparent to persons skilled in therelevant art(s) based on the following discussion. These steps aredescribed in detail below.

Flowchart 2700 begins with step 2702. In step 2702, a nanowire solutionis provided. For example, the nanowire solution is nanowire solution2210, shown in FIG. 22. As described above, in an embodiment, nanowiresolution 2210 is a liquid solution that contains a plurality ofnanowires.

In step 2704, the nanowire solution is directed through at least oneoutput opening of a nozzle onto the target surface. For example, FIG. 23shows a flow of nanowires 2302, which includes nanowire solution 2210,directed through openings 2304 a-e of nozzle 2202 onto target surface2204.

In step 2706, the nanowires are caused to become attached to the targetsurface. For example, step 2706 can include a step where nanowiresolution 2210 includes a curable material, and is cured on targetsurface 2204. In an embodiment, a charge can be applied to targetsurface 2204 causing electrostatic attraction to aid in causingnanowires from nanowire solution 2210 to stick to target surface 2204,and to remain in place, during the application of the nanowire solutionin step 2704. Charged polymers, chemicals, pigments, or agents can alsobe applied onto the surface. For example, a material can be applied to asubstrate to create a positively charged substrate. The positivelycharged substrate attracts negatively charged nanowires, such as siliconnanowires coated with an oxide (e.g., SiO₂). Materials such asaminopropyl triethoxy silane (APTES), polylysine positively chargedamine groups, an amine containing silane, or an amine containing polymercan be applied to a surface to produce this effect, such as onto asubstrate that contain silicon or other material.

In an embodiment, step 2704 includes a step where the nanowires arecaused to be substantially aligned parallel to each other on the targetsurface. For example, FIG. 24 shows target surface 2204 having aplurality of nanowires 2402 positioned thereon in alignment with eachother due to the present invention.

In an embodiment, step 2704 includes a step where the nanowire solutionis directed to overlapping portions of the target surface. For example,FIG. 24 shows nanowires 2402 positioned in a single distribution area.Alternatively, step 2704 can include a step where the nanowire solutionis directed to a plurality of non-overlapping portions of the targetsurface. For example, FIG. 25 shows nanowires 2502 positioned on targetsurface 2204 in a plurality of non-overlapping distribution areas 2504a-d.

In an embodiment, step 2704 can include a step where pressure is appliedto force the nanowire solution through the at least one output openingof the nozzle onto the target surface.

In an embodiment, target surface 2204 can be a very large surface, suchas a large area macro-electronic substrate. In such an embodiment, thetarget surface may be received as a continuous sheet on a conveyorbelt-type system, or other substrate supply system. Thus, in anembodiment, flowchart 2700 can include the step of adjusting theposition of the sheet relative to the nozzle. For example the sheet canbe supplied from a roll that continuously is fed past nozzle 2202. Suchrelative movement between nozzle 2202 and target surface 2204 can beused to align nanowires on target surface 2204, for example.

Reducing Phonon Scattering of Electrons in Nanowires Through Selectionof a Nanowire Semiconductor Material

Silicon is a suitable semiconductor material for nanowires, in part dueto its compatibility within the semiconductor industry. However, siliconhas disadvantages of a prevalence of surface states (˜10¹⁰ cm−2), andhas limits on the achievable nanowire wire diameters necessary toeliminate phonon scattering of electrons.

Thus, according to the present invention, alternative materials forfabrication of nanowires are disclosed. Such materials described hereinhave advantages, including reduced phonon scattering of nanowires andincreased diameter ranges.

There are at least two ways to reduce or eliminate phonon scattering ofelectrons. To reduce or eliminate phonon scattering in nanowires, theenergy spacing between the sub-bands of the nanowire material should be(1) larger than the phonon energy; and (2) larger than 3*k_(b)T, wherek_(b) is the Boltzmann constant (8.62×10⁻⁵ eV/° K), “3” is apredetermined factor, and T is the absolute temperature at which thenanowire device operates. For operation at room temperature, (2)typically is the more stringent requirement because the phonon energy ofmost semiconductor materials is less than 3*k_(b)T_(rt), where T_(rt) isroom temperature, and k_(b)T_(rt)=0.0259 eV, so that3*k_(b)T_(rt)=0.0777.

The quantum confinement of electrons in a semiconductor material relatesto the confinement of electrons in sub-bands within the conduction bandof the semiconductor material. Electrons in the material are confined tothe specific energy levels of the sub-bands. However, the electrons canmove from one sub-band energy level to another. The separation or energydifference between sub-bands can be calculated. For a nanowire materialwith isotropic electron (or hole) effective mass m_(eff), the energyspacing between the ground state and the first excited state is given by

$\begin{matrix}{{\Delta\; E} \approx \frac{8.9\;\hslash^{2}}{2\; m_{eff}r^{2}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$where:

-   -   h=Planck's constant÷2π    -   m_(eff)=effective mass of the selected semiconductor material;        and        -   r is the radius of the nanowire.            Quantum confinement of electrons in a semiconductor material            is maintained when electrons cannot scatter between            sub-bands. However, phonon scattering in a semiconductor            material causes electrons to be scattered between sub-bands            of the semiconductor material, which can reduce mobility of            electrons in the nanowire. To maintain quantum confinement            of electrons, phonon scattering of electrons must be            prevented in the semiconductor material, to prevent            electrons from being scattered between sub-bands. As            described below, quantum confinement of electrons can be            maintained by selecting semiconductor materials having a            known effective mass m_(eff), and forming nanowires from the            semiconductor material to have a diameters that deter            electrons from moving between energy levels of the sub-bands            due to phonon scattering.

The effective mass m_(eff) of different semiconductors makes some ofthem preferable to reduce or eliminate phonon scattering than others.Furthermore, nanowires having a smaller effective mass m_(eff) allow fora larger diameter. A larger diameter for a nanowire allows for bettercontrol during nanowire growth, and provides for additional strength ofthe nanowires.

By setting Equation 1 equal to Nk_(b)T_(rt), the following equation canbe formed, which can be used to calculate a maximum diameter for ananowire fabricated from a semiconductor material having an effectivemass m_(eff), to have substantially reduced or eliminated phononscattering:

$\begin{matrix}{\mspace{79mu}{{{{the}\mspace{14mu}{maximum}\mspace{14mu}{radius}} = \sqrt{\frac{(8.9)\hslash^{2}}{2\; m_{eff}{Nk}_{b}T}}}\mspace{79mu}{{wherein}\text{:}}\begin{matrix}{\hslash = {{Planck}\text{'}s\mspace{14mu}{{constant} \div 2}\;\pi}} \\{= {6.626 \times 10^{- 34}\mspace{14mu} J\text{-}{\sec \div \left( {2 \times 3.1416} \right)}}} \\{\left( {{or}\mspace{14mu} 4.14 \times 10^{- 15}\mspace{14mu}{eV}\text{-}{\sec \div 2} \times 3.1416} \right)} \\{{= {1.0545 \times 10^{- 34}\mspace{14mu} J\text{-}\sec\mspace{14mu}\left( {6.589 \times 10^{- 16}\mspace{14mu}{eV}\text{-}\sec} \right)}};} \\{{m_{eff} = {{effective}\mspace{14mu}{mass}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{semiconductor}\mspace{14mu}{material}}};} \\{{N = {a\mspace{14mu}{predetermined}\mspace{14mu}{factor}}};} \\{k_{b} = {{Boltzmann}\text{'}s\mspace{14mu}{constant}}} \\{{= {1.38 \times 10^{- 23}\mspace{14mu} J\text{/}{^\circ}\mspace{14mu} K\mspace{14mu}\left( {8.62 \times 10^{- 5}\mspace{14mu}{eV}\text{/}{^\circ}\mspace{14mu} K} \right)}};{and}} \\{{T = {{operating}\mspace{14mu}{temperature}}};} \\{{{wherein}\mspace{14mu}{at}\mspace{14mu}{room}\mspace{14mu}{temperature}},{{k_{b}T} = {4.144 \times 10^{- 21}\mspace{14mu} J\mspace{14mu}{\left( {{.0259}\mspace{14mu}{eV}} \right).}}}}\end{matrix}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$Thus, Equation 2 can be rewritten as:

$\begin{matrix}{{{the}\mspace{14mu}{maximum}\mspace{14mu}{radius}} = \sqrt{\frac{(8.9)\left( {1.0545 \times 10^{- 34}} \right)^{2}}{2\; m_{eff}{N\left( {4.144 \times 10^{- 21}} \right)}}}} \\{= \sqrt{\frac{1.194 \times 10^{- 47}}{{Nm}_{eff}}}}\end{matrix}$

Predetermined factor N is a factor that can be selected to provide astatistical assurance related to the confinement of electrons andreduction of phonon scattering. For example, predetermined factor N canbe selected to have any value greater than zero. In an embodiment,predetermined factor N is selected to have a value of greater than orequal (≧) to 3. In another embodiment, predetermined factor N isselected to have a value of greater than or equal (≧) to 5.

Graph 2800 shown in FIG. 28 shows a relationship between a maximumallowable diameter for a semiconductor material and effective massm_(eff). Maximum allowable diameter in nanometers (nm) for a nanowire isindicated on the Y-axis, and a normalized effective mass m_(eff)/m₀ isshown in the Y-axis of graph 2800. As shown in graph 2800, as thenormalized effective mass m_(eff)/m₀ decreases, a maximum allowablediameter for a nanowire increases. The maximum allowable diameter for ananowire increases sharply as normalized effective mass m_(eff)/m₀decreases below 0.2.

Table 2900 shown in FIG. 29 lists various example semiconductormaterials in a first column 2902. Listed in a second column 2904 are theeffective masses of the semiconductor materials of first column 2902,and listed in a third column 2906 are the energy gaps of thesemiconductor materials of first column 2902. As described above,semiconductor materials having relatively smaller effective mass m_(eff)are favorable, as they allow for larger nanowire diameters. GaAs, InSb(for infrared detectors), and InAs stand out as example favorablechoices in Table 2900, each having a relatively small effective massm_(eff), thus allowing for relatively larger nanowire diameters.

For example, applying the data of column 2904 to graph 2800 of FIG. 28,a maximum allowable diameter for a silicon (Si) nanowire havingsubstantially reduced or eliminated phonon scattering is ˜6 mn. Forgallium arsenide (GaAs) the maximum allowable diameter for a nanowirehaving substantially reduced or eliminated phonon scattering is ˜20 mn.

Furthermore, the data of table 2900 can be applied to Equation 2 tocalculate a suitable nanowire diameter for a particular semiconductormaterial. For example, as shown in column 2904 for GaAs, m_(eff)/m₀ isequal to 0.067, where m₀ is the free electron rest mass (9.11×10⁻³¹ kg).This value is applied to Equation 2, as follows:

$\begin{matrix}{{{maximum}\mspace{14mu}{radius}\mspace{14mu}{for}\mspace{14mu}{GaAs}} = \sqrt{\frac{1.194 \times 10^{- 47}}{N\frac{m_{eff}}{m_{0}}m_{0}}}} \\{= \sqrt{\frac{1.194 \times 10^{- 47}}{(3)({.067})\left( {9.11 \times 10^{- 31}} \right)}}} \\{\cong {8.07\mspace{14mu}{nm}}}\end{matrix}$ wherem₀ = free  electron  rest  mass = 9.11 × 10⁻³¹  kg; and N = 3.Thus, for GaAs, a diameter up to 16.14 nm can be used, while reducing oreliminating phonon scattering.

Thus, nanowires can be used individually, or in groups/thin films, wherethe nanowires are formed to have a diameter less than or equal to (≦) amaximum diameter determined for the semiconductor material of thenanowires(s) to allow each nanowire to retain substantial quantumconfinement of electrons (i.e., substantially reduce or completelyeliminate phonon scattering of electrons).

Furthermore, in an embodiment, each nanowire can be formed to have alength less than or equal to (≦) a predetermined length such thatballistic transport of electrons can occur through each nanowire.

FIG. 30 shows a flowchart 3000 providing example steps for designingconducting nanowires having high mobility of electrons, according to anexample embodiment of the present invention. Other structural andoperational embodiments will be apparent to persons skilled in therelevant art(s) based on the following discussion. These steps aredescribed in detail below.

Flowchart 3000 begins with step 3002. In step 3002, a semiconductormaterial is selected. For example, the semiconductor material can beselected from Table 2900 of FIG. 29, or can be any other semiconductormaterial described herein, or otherwise known.

In step 3004, a maximum diameter for a nanowire made from the selectedsemiconductor material is determined, that provides substantial quantumconfinement of electrons. For example, the diameter can be selected byviewing graph 2800 of FIG. 28, or by calculating the maximum diameterusing Equation 2.

In step 3006, a plurality of nanowires are formed from the selectedsemiconductor material, each nanowire of the plurality of nanowiresbeing formed to have a diameter less than or equal to (≦) to thedetermined maximum diameter.

Reducing Phonon Scattering of Electrons in Nanowires With DopingStrategies

According to the present invention, phonon scattering of electrons innanowires can be reduced or eliminated through nanowire semiconductordoping strategies. The effect of doping (whether n-type or p-type) canaffect the phonon scattering of electrons in the nanowire, as well asthe physical strength of the wire. In embodiments of the presentinvention, as further described below, semiconductors with eitherelectrons or holes as conducting carriers can be doped to improveperformance.

Most semiconductors have degenerate bands at the valence band maximum,with both heavy holes and light holes. The smallest energy spacingbetween the valence bands comes from the heavy holes with largereffective mass. Therefore, in a preferred embodiment, nanowires areconfigured so that electrons are the conducting carriers in thenanowires, because the effective mass of holes are greater than that ofelectrons.

Configuring nanowires to use electrons as conducting carriers can beaccomplished by n-doping the nanowires directly, and/or by applying abias gate voltage on p-doped wires. Table 3100 shown in FIG. 31 listsexample III-V semiconductor type materials in a column 3102. A column3104 contains corresponding effective masses for electrons for thematerials of column 3102. Columns 3106 and 3108 contain correspondingeffective masses for light holes and heavy holes, respectively, for thematerials of column 3102. For all the semiconductor materials listed inTable 3100, the effective mass shown in column 3104 for electrons isless than the effective masses contained in columns 3106 and 3108 forlight holes and heavy holes. Thus, n-doping of nanowires made from thesesemiconductor materials would lead to lower phonon scattering comparedto p-doping.

Note that Table 3100 listing III-V semiconductor type materials isprovided for illustrative purposes, and that the present invention isapplicable to semiconductor material types other than just III-Vsemiconductor types.

Furthermore, the mechanical strength of n-doped silicon is greater thanthat of p-doped silicon, which can also be an advantage in choosingn-doping of nanowires over p-doping of nanowires. For further details,refer to B. Busham and X. Li, Journal of Materials Research, Vol. 12,page 54 (1997).

Alternatively, the nanowires can be doped with a p-type dopant material,and the nanowires can be configured so that electrons are the conductingcarriers. For example, a thin film of nanowires incorporating thep-doped nanowires can be operated in an inversion mode by applying asufficient bias voltage to the thin film of nanowires. This can causeelectrons to be used as the conducting carriers in the p-dopednanowires, reducing or eliminating phonon scattering in the p-dopednanowires. For example, in a transistor embodiment, a bias voltage canbe applied to the plurality of nanowires as a gate bias voltage for thetransistor, to operate the nanowires in inversion mode.

Use of Core Shell Materials to Reduce Surface States in Nanowires

Another factor toward achieving increased mobility of electrons,including even ballistic transport of electrons in nanowires, is areduction in both surface and bulk (i.e., nanowire core) impurityscattering. In nanowires, surface scattering is especially significant,where a larger surface-to-bulk ratio exists.

Surface scattering can be reduced by forming an outer layer of thenanowire, such as by the passivation annealing of nanowires, and/or theuse of core-shell structures with nanowires. For example, FIG. 3B showsan example nanowire 310 doped according to a core-shell structure. Aninsulating layer, such as an oxide coating, can be formed on a nanowireas the shell layer. Furthermore, for example, for silicon nanowireshaving an oxide coating, the annealing of the nanowires in hydrogen (H₂)can greatly reduce surface states. In another example, for nanowireshaving compound semiconductors in the nanowire core, such as GaAs, theuse of an AlGaAs (or similar compound for other compound semiconductortypes) shell can effectively confine the electron wave functions, andalso serve to reduce surface states.

In embodiments, the a core-shell combination is configured to satisfythe following constraints: (1) the shell energy level should be higherthan the core energy level, so that the conducting carriers are confinedin the core; and (2) the core and shell materials should have goodlattice match, with few surface states and surface charges.

Note that in an embodiment, the thickness of the shell layer versus thewire diameter can be varied, to improve carrier mobility in thenanowires, and to reduce surface states.

FIG. 32 shows a flowchart 3200 providing example steps for fabricatingnanowires having reduced surface scattering, according to an exampleembodiment of the present invention. Other structural and operationalembodiments will be apparent to persons skilled in the relevant art(s)based on the following discussion. These steps are described in detailbelow.

Flowchart 3200 begins with step 3202. In step 3202, a semiconductormaterial is selected. Any semiconductor material, including thosedescribed elsewhere herein, or otherwise known, can be selected.

In step 3204, a plurality of nanowires are formed from the selectedsemiconductor material. For example, the nanowires can be grown orotherwise formed from the selected semiconductor material.

In step 3206, a circumferential surface of each nanowire of theplurality of nanowires is coated with an insulating layer. For example,in an embodiment, the insulating layer can be a dielectric material. Inanother embodiment, the insulating layer can be an oxide. Each nanowirecan be oxidized to form the insulating layer. In an embodiment, eachoxidized nanowire can be annealed. For example, each oxidized nanowirecan be annealed in an H₂ environment to passivate dangling bonds at theinterface of an oxidized layer and a non-oxidized portion of eachoxidized nanowire. For example, silicon nanowires can be oxidized tocreate an oxide layer of SiO₂. In another example, a nanowire formedfrom a compound semiconductor material can be oxidized in oxygen oroxygen combination that is conducive to forming an oxide on thematerial. For instance, GaAs nanowires could be oxidized in an oxygenand arsenic environment to create an oxide layer, such as a layer ofAs₂O₃. Other compound semiconductor materials can be similarly oxidized,or oxidized in other manners.

FIG. 33 shows another flowchart, flowchart 3300, providing example stepsfor fabricating nanowires having reduced surface scattering, accordingto an example embodiment of the present invention. Other structural andoperational embodiments will be apparent to persons skilled in therelevant art(s) based on the following discussion. These steps aredescribed in detail below.

Flowchart 3300 begins with step 3302. In step 3302, a semiconductormaterial is selected. Any semiconductor material, including thosedescribed elsewhere herein, or otherwise known, can be selected.

In step 3304, a plurality of nanowires are formed from the selectedsemiconductor material. For example, the nanowires can be grown orotherwise formed from the selected semiconductor material.

In step 3306, each nanowire of the plurality of nanowires is doped sothat each nanowire comprises a core-shell structure, wherein the shellis a doped outer layer of each nanowire surrounding a respective core ofeach nanowire. For example, the nanowire is nanowire 310, shown in FIG.3B. The doped outer layer is doped surface layer 302. In an embodiment,an effect of step 3306 is to cause carriers of each nanowire to besubstantially confined to the axially-located core during operation.

In an embodiment, step 3306 can include the step where a dopant materialis selected for the doped outer layer of each nanowire such that thedoped outer layer would have a higher energy level relative to an energylevel of the respective core.

In an embodiment, step 3306 can include the step where a dopant materialis selected for the doped outer layer so that a lattice structure of thedoped outer layer substantially matches a lattice structure of the core.

Nanowire And Nanoribbon Thin Film Transistors

FIGS. 34A-34C illustrate concepts related to high mobility nanowire andnanoribbon TFTs. FIG. 34A represents amorphous (a-Si) or polycrystallineSi (poly-Si) TFTs. As can be seen from the FIG. 34A, electrical carriershave to travel across multiple grain boundaries resulting in low carriermobility. FIG. 34B illustrates a NW-TFT, according to an embodiment ofthe present invention. Unlike a-Si or poly-Si TFTs in which carriershave to travel across multiple grain boundaries resulting in lowmobility, NW-TFTs have conducting channels formed by multiple singlecrystal NW paths (like a log bridge) in parallel. Thus, electrons/holestravel within single crystals all the way between the source to drainelectrodes, with high carrier mobility. Similarly, as depicted in FIG.34C, according to an embodiment of the invention, single crystalsemiconductor nanoribbons, which have characteristics similar to that ofmultiple nanowires in parallel, are also used to produce TFTs with highperformance.

NW-TFT Device Fabrication

FIG. 35A illustrates a flowchart 3200 providing example steps forfabricating NW-TFTs, according to an embodiment of the invention. Otherstructural and operational embodiments will be apparent to personsskilled in the relevant art(s) based on the following discussion. Thesesteps are described in detail below.

Flowchart 3500 begins with step 3510. In step 3510, single crystalnanowires are synthesized. For example, p-type silicon nanowires withcontrolled diameters can be synthesized by decomposition of SiH₄ andB₂H₆ using gold colloid particles (available through British BiocellInternational Ltd., for example) as the catalyst in a pilot productionscale reactor. In an example embodiment, the growth can be carried outat a temperature between 420-480 EC, with a total pressure of 30 torr,and a silane partial pressure of approximately 2 ton, for a period of 40minutes. The SiH₄ and B₂H₆ ratio can be varied to control the dopinglevel. A ratio of 6400:1 is used in synthesizing nanowires for whichmeasurements are provided below, although other ratios can be used. Inan example application of step 3510, nanowires synthesized by thisprocess had lengths in the range of 20-40 μm, with a nearlymono-dispersed diameter as determined by the Au colloid catalyticparticle. In the current example application, the nanowires weresynthesized to have a core shell structure with a single crystallinesilicon core surrounded by an amorphous silicon oxide shell of thickness2-3 nm.

In step 3520, the nanowires are suspended in solution. In an exampleembodiment, the synthesized nanowires can be transferred into ethanolusing ultra-sonication to obtain a stable nanowire suspension.Alternatively, nanowires can be transferred into, and suspended in othersuitable solution types.

In step 3530, the nanowires are assembled into a thin film. In thepresent example, after being dispersed into solution, the nanowires wereassembled onto a chosen substrate using a fluidic flow directedalignment method to obtain an oriented nanowire thin film. The nanowiresuspension was allowed to pass through a fluidic channel structureformed between a poly-dimethlysiloxane (PDMS) mold and a flat substratesurface to obtain NW arrays on the surface. The average NW space in thethin film was controlled by varying the NW concentration in the solutionand/or the total flow time. With this approach, the alignment can bereadily extended over a 4-inch wafer or even larger areas by using alonger or larger flow channel mold. An example such fluidic channelstructure is described above, as flow mask 1500 shown in FIGS. 15A and15B.

FIG. 35B shows an optical micrograph of an example nanowire thin havinga single layer of nanowires oriented in parallel, with an averagenanowire spacing of about 500-1000 nm. The micrograph of FIG. 35Bfurther shows that a few nanowires cross over the top of the nanowirethin film, although in alternative embodiments, thin films can becreated with no nanowires crossing over the top of the nanowire thinfilm. The scale bar illustrated in FIG. 35B is 100 μm in length. Aninset 3502 within FIG. 35B provides higher magnification and includes ascale bar of 20 μm.

In step 3540, a thin film transistor device is fabricated. The thin filmtransistor device incorporates the nanowire thin film created in step3530. Any type of thin film transistor device can be fabricated.

In step 3550, thin film transistor devices are integrated for theparticular macroelectronic application. In an example embodiment, a NWthin film can be subjected to standard photolithography or E-beamlithography processes to define source and drain electrodes and yieldTFTs on a macroelectronic substrate.

FIG. 35C illustrates an example TFT, where the scale bar is 100 μm.Different materials can be used for electrodes, including goldelectrodes, which are shown for the example TFT device in FIG. 35C aslight colored bars. For the example TFT 3504 shown inset in FIG. 35C, aback-gated device configuration on a silicon substrate is shown.Underlying silicon is used as the back gate, silicon nitride (SiN_(x))100 nm thick is used as the gate dielectric, and a Ti/Au (60 nm/60 nm)film deposited using an e-beam evaporator is used as the source anddrain electrodes.

FIG. 35D provides an optical micrograph of a NW-TFT that shows parallelarrays of NWs bridging the source and drain electrodes 3506 and 3508.The scale bar in FIG. 35D is 5 μm in length.

Step 3560 is the end of flowchart 3500.

Performance Characteristics—P-Channel Silicon Nanowire Thin FilmTransistors (NW-TFT)

In the following example, electric characterization of NW-TFTs wascarried out in an air environment, in a dark box, at room temperature.FIG. 36A shows a plot of drain current (I_(DS)) versus drain-source biasvoltage (V_(DS)) curves at various gate voltages (V_(GS)), which arevaried in steps of 1 volt (V), beginning with the upper curve havingV_(GS)=−10V. In this example, the TFT consists of ninety-onesubstantially parallel 20 nm diameter nanowires, and has a 5 μm channellength. FIG. 36A shows typical p-channel transistor behavior operatingin accumulation mode, as the drain current I_(DS) increases linearlywith V_(DS) at low V_(DS), and tends to saturate at higher V_(DS). Uponapplication of negative gate voltages, the drain current increases asthe majority carriers (holes) increase in the channel. Applying apositive gate bias depletes holes in the channel and turns the deviceoff.

A plot of I_(DS) versus V_(GS), illustrated in FIG. 36B, at a constantV_(DS)=−1 V, shows that essentially no current flows when the gatevoltage is more positive than a threshold point, and the I_(DS)increases nearly linearly when the gate voltage increases in thenegative direction. Linear extrapolation of the linear region results ina threshold voltage of 0.45 V. An inset plot 3602 shown within FIG. 36Bshows −I_(DS) versus V_(GS) at V_(DS)=−1V in the exponential scale. FIG.36B highlights an on-off ratio of nearly 10⁸, and a sub-threshold swingof about 500 mV/s. The linear plot data were collected at a V_(GS) sweeprate of 500 mV/s, and the exponential plot data were collected at aV_(GS) sweep rate of 15 mV/s to minimize capacitive charging currents atthe higher gate voltages. The apparent threshold voltage in the inset isshifted to 3.5V due to a hysteresis effect.

A hysteresis effect is commonly observed in I_(DS)-V_(GS) relation ofthe current for NW-TFTs. This hysteresis is likely primarily due to themobile ions present in the NW-TFT devices. This hysteresis can beeliminated or minimized by stringent control of NW synthesis and devicefabrication processes, to minimize ion contamination. The hysteresis hasan effect in determining threshold voltages. Due to hysteresis, theapparent threshold voltage can vary depending on the measurementcondition and the voltage history that the device experienced beforemeasurements.

In order to minimize the threshold voltage variation caused byhysteresis, similar conditions were used (relatively quick gate voltagesweeping rate of 500 mV/s was used to minimize the mobile ion effect) totest devices. Voltage history variation was also minimized by firstcycling the gate voltage (from 10 to −10V) at least three times beforecollecting data for each device. In this way, a justified thresholdvoltage distribution (FIG. 36B main plot and FIG. 36C) could bedetermined. On the other hand, to accurately measure off-state current,a slower gate voltage sweep rate (15 mV/s) to minimize the capacitancecurrent was used. In this case, the device experienced high positivegate voltage for extended period of time (about 5-10 min) and shiftedthe apparent threshold to a more positive value (inset FIG. 36B).

For macroelectronic applications, a number of key transistor parameters,including transconductance, mobility, on/off current ratio, thresholdvoltage, and sub-threshold swing, dictate the TFT performance. A hightransconductance is important to a transistor's performance, and tovoltage gains of transistor-based devices, including amplifiers andlogic circuits. The slope in the linear region of −I_(DS) versus V_(GS)shows a transconductance, g_(m)=dI_(DS)/dV_(GS), of about 11 μS atV_(DS)=−1 V. Assuming the effective channel width equals to the NWdiameter, d, multiplied by the number, N, of NWs: W_(eff)=N·d=1.8 μm.For comparison to a-Si TFTs and poly-Si TFTs, a normalizedtransconductance of about 6 μS/μm can be obtained. This is significantlybetter than that of amorphous-Si TFTs, which have transconductances ofabout 0.01 μS/μm, and of p-channel poly-Si TFTs, which havetransconductances of about 0.3 to 0.8 μS/μm. Furthermore, this iscomparable to that of single crystal p-channel silicon-on-insulator(SOI) MOSFETs, which have transconductances of about 5 to 12 μS/μm. Notethat the NW TFT transconductance can further be improved using thinnerdielectrics of higher dielectric constants.

Additional modeling using standard MOSFET equations was conducted tofurther estimate the carrier mobility in the NW-TFT under study. In thelow-bias linear region of I_(DS)-V_(DS) curves, the hole mobility μ_(h)can be deduced from:G _(DS) =I _(DS) /V _(DS)=μ_(h) C _(G)(V _(GS) −V _(th) −V _(DS)/2)/L ²,

Where:

-   -   C_(G) is the gate capacitance, and    -   L is the channel length.        The gate capacitance includes the capacitance of the SiNx        dielectric on the substrate and that of the silicon oxide shell.        It is, however, nontrivial to calculate these capacitances.        Simulation using a 3D finite element package (e.g., Metamesh and        HiPhi from Field Precision, http://www.fieldp.com) yields a        total capacitance of about 27 fF, leading to a hole-mobility        about 130 cm²/V·s. This hole-mobility is higher than the best        value reported for p-type polysilicon (about 120 cm²/V·s) and        comparable to that of p-type single crystal silicon material,        such as SOI MOSFET (about 180-250 cm²/V·s). In addition, it is        possible that the carrier mobility can be further improved, for        example, by decreasing the doping level and/or minimizing the        trapping states on NW surface.

Inset 3602 in FIG. 36B shows a plot of the I_(DS)−V_(GS) curve in theexponential scale, showing that the drain current decreasesexponentially below the threshold voltage and that the transistor has anon-off current ratio close to 10⁸. This represents the largest on-offratio reported for transistors assembled from chemically-synthesizednanomaterials and is comparable to that of single crystal silicondevices. The exponential decrease in current defines a key transistorparameter, the sub-threshold swing S=−dV_(GS)/dIn|I_(DS)|, to be about600 mV/decade in this device. In conventional MOSFETs, S is determinedby S=(k_(B)T/e)·Ln{(10)(1+α)} where T is temperature, k_(B) isBoltzmann's constant, e is elementary charge, and α depends oncapacitances in the devices and is 0 when the gate capacitance is muchlarger than other capacitances such as interface trap state capacitance.The lowest theoretical limit for S is therefore S=(k_(B)T/e)·Ln(10), orabout 60 mV/decade at room temperature.

In general, a small sub-threshold swing is desired for low thresholdvoltage and low-power operation. A sub-threshold swing of about 600mV/decade in an embodiment of the NW device of the present invention issignificantly better than conventional amorphous Si TFTs or organicsemiconductor TFTs, which typically range from one to many volts perdecade. Furthermore, this is comparable to most poly-Si TFTs, but issubstantially larger than the best poly-Si TFT (about 200 mV) and singlecrystal silicon devices (about 70 mV).

The relatively large sub-threshold swing observed the NW device of theinvention is likely mainly due to the existence of surface trappingstates and a geometric effect, which can be improved dramatically bypassivating the surface (e.g., hydrogenation or using a core-shellstructure) and/or using top- or surrounding-gated structure with high-kdielectrics.

A geometric effect results from NWs crossing over other NWs in theNW-TFT. The NW thin film typically consists of a near monolayer of NWs,but occasionally a few NWs cross over other NWs. When a NW crosses overother NWs, it is separated from the substrate surface, experiences asmaller electrical field from the back gate, and thus turns on or offmore slowly than other NWs in the device. This increases thesub-threshold swing of the NW-TFT as a whole. Nonetheless, andimportantly, a sub-threshold swing as small as about 70 mV/decade hasbeen demonstrated using a surrounding conformal electrolyte gate, asdiscussed below.

In practical applications, the variability in threshold voltage fromdevice to device is a key factor in determining the viability of atechnology. To this end, tests have been conducted on 20 individualdevices that were fabricated according to an embodiment of theinvention. FIG. 36C shows a histogram of the threshold voltagedistribution of these devices. Gaussian fitting shows a standarddeviation of only 0.22V. Additionally, optimization of the configurationand fabrication process is likely to lead to tighter distributions.

Specifically, the performance of NW-TFTs can be further improved in anumber of ways by exploiting various NW core-shell structures. First, inNW-TFTs on plastic, the on-off ratio is limited by the low-qualitye-beam evaporated AlO_(x) gate dielectrics. This problem can bepotentially overcome by using a core-shell NW structure consisting of asingle crystal semiconductor core and a high quality gate dielectricshell. Although Si NWs naturally have a core-shell structure, the thinnative oxide layer is not high enough quality to withstand a highelectric field. The native oxide can be replaced or supplemented with ahigh quality silicon oxide shell generated by either controlled thermaloxidation, chemical vapor deposition, or other suitable technique.Core-shell NW structures are very well suited for making highperformance NW-TFTs on plastic because high temperature processes,including semiconductor material synthesis and high quality gatedielectric formation, are performed separate from the final devicesubstrate. In addition, such a core-shell structure can also lead topassivation of surface trapping states, resulting in further performanceenhancement.

Second, the current back-gated NW-TFTs are relatively limited inperformance due to a geometrical effect. Such a geometrical effect canbe overcome by using a more complex NW core-shell structure to include acore of single crystal semiconductor, an inner-shell of gate dielectric,and an outer-shell of conformal gate. This can be realized by depositinga layer of highly-doped amorphous silicon around the Si/SiO_(x)core-shell structure (described above) as the outer-gate shell, forexample.

Third, the performance of NW-TFTs can potentially be further improved toexceed that of single crystal materials by exploiting the quantumelectronic effect in small diameter NWs. In analogy to conventional twodimensional semiconductor superlattices and 2D electron/hole gas,multi-core-shell NW structure can be envisioned to separate the dopantsfrom the active conducting channel to achieve ultra-high mobility TFTs.

Major parameters of device performance (such as carrier mobility andthreshold voltage) are independent of the number of NWs in theconducting channel. Thus, the design and fabrication of NW-TFTs withpre-defined characteristics are possible. For example, by varying thenumber of NWs in the conducting channel (changing effective channelwidth), NW-TFTs can be designed and fabricated to carry current at apredetermined level. FIG. 36D illustrates the linear-scale relation forthe drain current when the device is turned on (Vgs=−10V). The twocurves show the on-state current as a function of effective channelwidth. The lower curve has V_(DS)=−1V and the upper curve as V_(DS)=−8V.The effective channel width corresponds to the product of the averagediameters of the NWs and the number of NWs in the channel. As expected,the on-state current scales linearly with the effective channel width(number of NWs in conducting channel). An on-state current of more than0.5 mA has been achieved from a device with an effective channel widthless than 5 μm. Reproducible and predictable assembly of NW-TFTs withdesigned device parameters is important.

NW-TFTs on Plastics

One important aspect of the current NW thin film concept is that theentire NW-TFT fabrication process can be performed essentially at roomtemperature, except for the NW synthesis step, which is separate fromthe device fabrication. Therefore, the assembly of high performanceNW-TFTs can be readily applied to low cost glass and plastic substrates.To demonstrate NW-TFTs on plastic substrates, a different deviceconfiguration is used. FIG. 37A illustrates the device configuration. Tofabricate the device of FIG. 37A, a layer of 1-2 μm thick SU-8(MicroChem Corp.) photo-resist is first spin cast and cured on apolyetheretherketon (PEEK) sheet (50 or 100 μm thick, Goodfellow Inc.)to ensure a microscopically smooth surface. Cr/Au (10/30 nm) strips aredefined as the gate arrays, and a 30 nm layer of aluminum oxide isdeposited as gate dielectric using e-beam evaporation. The aligned NWthin film is deposited on the surface, and Ti/Au (60/80 nm) source-drainelectrodes are defined to form the TFTs. FIG. 37B shows example plasticdevices with NW-TFTs. The lower portion of FIG. 37B illustrates thesource, gate, and drain of the NW-TFT with a scale bar equal to 5 μm.

Electrical transport characterization was performed in the same way asdescribed above. FIG. 37C illustrates that the I_(DS)−V_(DS) curves showa similar behavior to that of devices on SiN_(x)/Si substrate. The plotof FIG. 37C shows an I_(DS)−V_(DS) relationship, with variable V_(GS),starting from the top at V_(GS)=−8V and increasing in the steps of 1V.The I_(DS)−V_(GS) relationship shows a threshold voltage of about 3.0V,and an on-off ratio >10⁵.

FIG. 37D illustrates transfer characteristics of the same device beforeand after slight flexing of the plastic substrate. An inset 3702 of FIG.37D highlights an on-off ratio of more than 10⁵ and a sub-thresholdswing of 500-800 mV/decade. The device under test had 17 NWs of 40 nmdiameter in parallel with a 6 μm channel length and a 3 μm gate length.The two curves show transfer characteristics for the same device beforeand after slight flexing of the plastic substrate (radius of curvatureof about 55 nm), demonstrating the mechanical flexibility of NW TFTs onplastics.

A relatively smaller on-off ratio (compared to that of the devices onSiN_(x)/Si substrates) is due to: (1) lower on-current due toun-optimized local-gate device configuration, (2) higher off-currentlimited by gate leakage current caused by the low-quality of e-beamevaporated AlO_(x) dielectrics; and can be significantly increased withimproved device configuration and an advanced core-shell NW structure.

The reduced sub-threshold swing primarily results from two factors.First, the electrolyte solution makes an excellent conformal gate andtherefore eliminates or reduces any geometrical effect that broadens thesub-threshold swing. Second, with an electrolyte solution conformalgate, the gate capacitance is more than one order of magnitude largerthan that of back gated devices (about 0.77 pF versus about 0.05 pF fora device of about 160 NWs). Therefore, the relative importance of othercapacitance is reduced as compared to gate capacitance, leading to adramatic decrease of a value in S=(k_(B)T/e)Ln(10)(1+α), andconsequently a decrease of sub-threshold swing S.

A major motivation driving plastic-electronic research is mechanicalflexibility. Importantly, a slight flexing of the plastic with NW-TFTdevice does not significantly change device behavior. As discussedabove, FIG. 37D provides two curves that illustrate this feature. Asillustrated in FIG. 37C, the linear region in the I_(DS)−V_(GS) relationgives a transconductance of 0.45 μS at V_(DS)=−1V. However, it isdifficult to estimate an exact hole-mobility in a device because ofdifficulties in estimating a gate capacitance in the local-gated deviceconfiguration.

In order to gauge carrier mobility and ultimate device performance ofNW-TFTs on plastics, an electrolyte gated TFT structure has been tested.FIG. 38A illustrates an electrolyte gated NW-TFT structure that wastested. This approach was used to study individual carbon nanotube FETs.The testing approach entailed the following steps. A small drop of 1 mMsalt solution was placed on a TFT device on a plastic substrate,covering the whole TFT device including the source, drain electrode, NWthin film and an additional isolated gold electrode. A voltage V_(GS)was applied to the isolated gold electrode to establish anelectrochemical potential in the electrolyte relative to the NW-TFTdevice. For a voltage range of less than ±0.9V, the leakage currentbetween electrolyte solution and the source, drain electrodes or NWs wasnegligible. The electrolyte functions as a well insulated liquid gate.Of particular merit, the electrolyte solution makes a useful surroundingconformal gate for all the NWs in the TFT channel, reducing oreliminating undesirable geometric effects and surface charges, thusproviding an effective configuration to test the ultimate performance ofthe NW-TFTs.

FIG. 38B shows a I_(DS)−V_(DS) relationship as a function of variouselectrolyte solution gate voltages in steps of 0.1 V, beginning with thetop curve having V_(GS)=−0.9V. The NW-TFT tested included 162 20 nmdiameter NWs in parallel with a Sum channel length.

FIG. 38C shows an I_(DS)−V_(GS) relationship for a V_(DS) of 10 mV. Theoverall results resemble those of TFTs fabricated on silicon substratesas described above. Notably, however, in this device the drain currentI_(DS) is much more responsive to changes in the electrolyte gatevoltage. The sub-threshold swing is also significantly reduced (70-110mV/decade), as shown in inset 3802 of FIG. 38C. The I_(DS)−V_(GS)relationship was determined with a lock-in amplifier (available throughStanford Research, for example). A 10-Hz sine wave at 10-mV RMSamplitude was used for the measurement.

In addition, the particular architecture of the solution gatingexperiment can underestimate the performance of an ideal device, sincethe source and drain electrode are also in contact with the solution.The source and drain potentials can affect the actual solution potentialand compromise the electrochemical potential established by the gateelectrode, decreasing the actual applied potential around the NWs tobelow that applied to the gate electrode. Therefore, with an improvedapproach (e.g., using standard reference electrode) to establish thegate potential, it is possible to achieve even smaller sub-thresholdswings. Together, the results demonstrate the potential for highperformance TFTs on plastics, which can be improved by furtheroptimizing the gate configuration of these devices (e.g., a solid statesurrounding conformal gate can be achieved in a multi-core-shell NWstructure consisting of single crystalline semiconductor core,inner-shells of gate dielectrics and outer-shell of conformal gates).

The performance of the electrolyte gated TFT device on plastics wasfurther analyzed by examining various characteristics of devicefeatures. The gate capacitance was first examined. A total capacitance,in this case, includes the series capacitance of the electrolytesolution and the capacitance of the NW shell oxide, although the formercan be ignored since it is much larger than the later. NWs having asingle crystalline core of 20 nm and an average of approximately 2.5 nmamorphous silicon oxide shell were considered. The gate capacitance canbe estimated by C_(G)=2Nπ∈∈₀L/ln((1+t_(ox)/r), where N is the number ofNWs in the channel, ∈ and t_(ox) are a dielectric constant and athickness of the amorphous oxide respectively, and r is a radius of theNWs. Based on the calculated capacitance and the following expressionG_(DS)=I_(DS)/V_(DS)=μ_(h)C_(G)(V_(GS)−V_(th)−V_(DS)/2)/L², the holemobility, μ_(h), can be determined to be about 150 cm²/V·s. This resultagrees with the mobility obtained for similar devices on SiN_(x)/Sisubstrate having similar NWs. This shows that the mobility of the NW-TFTis intrinsic to the NWs themselves, and not dramatically affected by theplastic substrate or the electrolyte gate.

N-Channel CdS Nanoribbon TFT

The above discussion demonstrates that high performance NW-TFTs can beassembled on low temperature plastic substrates from p-type Si NWs.Furthermore, since NW synthesis is independent of the final devicesubstrate, a broad range of materials including III-V and II-VI groupsemiconductors can be exploited as the TFT channel materials, creating abroad range of opportunities. As one example, high performance TFTs canalso be readily assembled from II-VI group cadmium sulfide (CdS)nanoribbons. CdS is an excellent material for optical as well aselectronic applications due to its intrinsically low surface trappingstates. Single crystal CdS nanoribbons with a thickness of 30-150 nm,width of 0.5-5 μm, and length up to 10-200 μm were synthesized using avacuum vapor transport method.

Specifically, a small amount of CdS powder (about 100 mg) wastransferred into one end of a vacuum tube and sealed. The vacuum tubewas heated such that the end with CdS powder was maintained at 900° C.,while the other end of the vacuum tube was kept at a temperature about50° C. lower. Within two hours, most of the CdS was transported to thecooler end and deposited on the tube wall. The resulting materials arepredominantly nanoribbons having thicknesses of 30-150 nm, widths of0.5-5 μm, and lengths of 10-200 μm. TEM images show these nanoribbonsare single crystals with low defects all the way to the edge surface.

Nanoribbons are useful for TFTs since their unique physical morphologyclosely resembles that of conventional single crystal thin film. CdSnanoribbon TFTs with a single-crystal conducting channel were fabricatedusing an approach similar to that described above. FIG. 39A illustratesa CdS nanoribbon TFT. The inset 3902 of FIG. 39A shows a 3D atomic forcemicroscopic topographic image of a nanoribbon TFT.

Electrical transport measurements for the CdS nanoribbon TFT showtypical n-channel transistor characteristics. The n-channel behavior isconsistent with previous studies on CdS bulk materials and NWs. FIG. 39Bprovides the I_(DS)−V_(DS) relation at different gate voltages for a CdSnanoribbon TFT. FIG. 39B shows a linear region at low source to drainbiases and saturates at a higher bias. The I_(DS)−V_(GS) relation at aV_(DS) of 1 V shows nearly linear behavior above a threshold V_(GS) of2.0V. The slope in the linear region gives a transconductance of about2.4 μS/μm at V_(DS)=1V. Assuming a parallel plate model, the gatecapacitance was calculated to be 1.9 fF using C_(G)=∈∈₀L·W/h, where Land W are channel length and width, and h is the dielectric thickness.With the calculated capacitance, an electron mobility can be deduced tobe about 300 cm²/V·s usingI_(DS)/V_(DS)=μ_(e)C_(G)(V_(GS)−V_(th)−V_(DS)/2)L². Importantly, thismobility value matches closely with that of single crystal CdS material(about 300-350 cm²/V·s).

Furthermore, the exponential plot of I_(DS)−V_(GS) gives an on-off ratiogreater than 10⁷ and a sub-threshold swing, S, as small as 70 mV/decade,as illustrated in FIG. 39C and its inset, approaching the theoreticallimit of 60 mV/decade. The high carrier mobility and small sub-thresholdswing observed in CdS nanoribbon TFTs can be largely attributed to highcrystalline quality and the low-surface states in these materials aswell as the absence of geometrical effects like that in Si NW-TFT.

Complementary Logic

The ability to fabricate both p- and n-channel TFTs is critical toconstruct complementary electronics that are known to be superior inperformance to circuits consisting of unipolar p- or n-channeltransistors. To this end, a complementary inverter (a logical NOT gate)was constructed by connecting an n-channel and a p-channel TFT inseries. The complementary inverter was formed by connecting a p-channelSi NW-TFT (consisting of 15 NWs in parallel) and n-channel CdSnanoribbon TFT in series. The device 4002 is illustrated in FIG. 40.FIG. 40 also provides the output-input (V_(out)-V_(in)) voltage responseof the inverter, and shows constant high voltage output with low input.When the input is increased to about 1.5 V, the output quickly turns to0 V and maintains a low state at higher input voltages. Mostsignificantly, the complementary inverter exhibits a high voltage gain.Differentiation of the measured V_(out)-V_(in) relation reveals avoltage gain as large as 27, as illustrated as inset 4004 in FIG. 40.Such a large gain demonstrates high performance of our devices and willbe critical for interconnection of arrays of logic circuits for avariety of large area electronic applications without the need forsignal restoration at each stage. Lastly, it should be noted thatV_(out)-V_(in) relation of the inverter was measured without any load onthe output. The gain may decrease when the device is loaded in an actualcircuit. However, with careful device/circuit design, the desiredvoltage gain in practical applications should be attainable, consideringthe reproducibility and predictability of the NW-TFT devices describedherein with characteristics, such as those shown in FIGS. 36A-D.

NW-TFTs in Display and Other Applications

Over the last twenty years, Flat-panel displays (FPDS) have become morecommonplace in modern electronic devices. FPDS are indispensable in manynew products, including cellular phone, personal digit assistant, digitcamera, camcorder, and notebook computer. In addition, the market isexpected to expanding significantly since FLD is poised to replacedesktop computer and television (TV) cathode ray tube (CRT) monitors.Active matrix liquid crystal display (AMLCD) is the dominant commercialflat-panel display technology, dominating nearly the entire large areaflat-panel display market. AMLCD is also sometimes referred to as activematrix thin film transistors (AMTFTs) because large area thin filmtransistors are the key technology which enable today's AMLCD.

The thin film transistor (TFT) was invented 13 years before the pointcontact junction transistor. The first US patent on the TFT was issuedto Lilienfield in 1933. As late as earlier of 1960's, many industrialresearch lab, including GE, RCA, IBM, Zenith, Westinghouse and Philips,were actively engaged in TFT research and development. However, ataround the middle of 1960's, metal-oxide semiconductor field effecttransistor (MOSFET) came along, and became the focal point. Soon, mostindustrial laboratories dropped TFT research and development. Becausethe majority of today's semiconductor technologies, including MOSFETtechnology, are single-crystal wafer based, the size of a substrate isdetermined by the size of wafers available. To date, the biggest waferavailable is ˜12″. Hence, this substrate size is probably not suitablefor applications that require large substrate areas.

The emergence of liquid crystal displays (LCDS), in particularly,AMLCDS, in the middle of 1980's, which require driving circuitry to belocated on a large glass substrate, renewed the interest in the TFTtechnology. Earlier efforts of the TFT were focused on II-VIsemiconductor materials. The technology never went beyond the researchlab due to difficulties in control of the II-VI semiconductor materials.For example, in general it is more difficult to make crystallized phaseof a compound semiconductor than that of an element. Furthermore, thedoping of II-VI materials, such as CdSe, is difficult. It was also verydifficult to deposit reliable dielectric materials on II-VI materials.

At the same time, hydrogenated amorphous silicon (a-Si:H) thin film hasattracted great attention due to its potential application as a solarcell and as imaging sensor materials. A turning point came in 1975, whenW. E. Spear and P. G. LeComber demonstrated that amorphous siliconmaterial could be doped. Very soon, TFT based on a-Si:H became thechoice for driver elements of AMLED despite poor transistor properties.The technology is almost exclusively used in today's large screencommercial AMLCD displays. In a AMLCD display, the a-Si TFTs arefabricated on a glass substrate underneath the LCD pixels, and are usedas switches to turn pixels on/off upon receiving the command from aintegrated circuit (IC) driving circuit. The IC driving circuit ismounted on the periphery of the substrate. The a-Si thin films can beeasily deposited on relatively large glass substrates at low temperatureusing plasma assisted chemical vapor deposition methods. The lowdeposition temperature makes using cheap glass substrate possible. Theglass substrate is a necessity because of transparency of the substrateis a must for the back-site illumination technique of the technology.

The field effect mobility of a typical a-Si FET is around ˜1 cm²/V·s,which limits the performance of the display. Extensive effort has beendevoted worldwide towards improving the performance of a-Si TFT bycrystallizing a-Si into polycrystalline thin film. The field effectmobility of polysilicon TFT lies in between that of a-Si TFTs and singlecrystal silicon transistors, with values up to several hundredsreported. Current polycrystalline processes typically require annealinga-Si at 600° C. for up to 24 hrs in order to produce transistors with amobility of 10-50 cm²/Vs. In addition to direct thermal annealing,several methods, including rapid thermal annealing, laser inducedcrystallization, and transition metal induced annealing have beenexplored to convert a-Si film to polycrystalline. The rapid thermalannealing uses higher temperatures, from 700° C. to 800° C., for veryshot periods of time. The short time duration minimizes potential damageto the substrate. However, it is unlikely that inexpensive glasssubstrates can be used for this process. Laser annealing allows smallareas of amorphous silicon to be heated up to very high temperaturerapidly without significantly heating the substrate. Unfortunately, dueto a small beam size, the method is very inefficient for large scaleproduction. Additionally, the process is very difficult to control.

Metal induced crystallization has attracted great attention recently.Nickel based processes appear to be promising. In general, a nickelbased process reduces the annealing temperature required for a slowthermal process from approximately 600° C. to between 500° C. and 550°C., with a duration of annealing reduced from ˜24 hrs to a few hours.However, metal induced crystallization requires an extra step of thetransition metal deposition on the top of the a-Si. Crystallization isdependent of the quality of metal film. Residue metal, metal silicidesand structural defects of complicate grain boundaries can result in highleakage current in transistors.

Poly TFT is not likely to replace a-Si technology soon because there isstill no viable polysilicon process that is compatible with inexpensiveglass substrates. The performance of polysilicon TFTs is not likely tosoon approach those of devices fabricated from traditional singlecrystal silicon because of the grain boundary conduction and difficultyin the preparation high quality polysilicon. Therefore, the currentlyavailable TFT technologies based on a-Si or p-Si are limited fromvarious perspectives.

Recently a new thin film transistor technology—organic TFT—has attractedmuch attention. Organic TFTs with field effect mobility up to ˜1 cm²/V·shave been demonstrated. By its nature, organic transistors arecompatible with low temperature processes on a plastic substrate,although most of work has been carried out using glass or oxide coatedsilicon as the substrate. Unfortunately, however, the performance oforganic transistors does not currently approach that of silicon. Hence,the area for application of organic based transistors is limited.Attempts to place silicon on plastics has not yet yielded satisfactoryresults, mainly because the required temperature of two key steps,silicon deposition and gate dielectric materials deposition, even forfabrication of amorphous silicon transistors, is too high for plasticsubstrates with the highest glass transition temperature to date towithstand.

A recent breakthrough in Prof. Charles Lieber's lab at HarvardUniversity, carried out substantially by the principal investigator ofthis program, Dr. Xiangfeng Duan, have shown that semiconductornanowires are excellent or ideal building blocks for nanoscaleelectronics and optoelectronics. Prof. Lieber's lab have demonstratedthat a wide range of group IV, III-V and II-VI semiconductor nanowirescan be rationally synthesized in single crystal form with controlled andtunable chemical composition, physical dimension (e.g., diameter andlength), and electronic properties (e.g., doping type andconcentration). The diameter of the nanowires can be controlled andvaried in the range of 2-100 nm. The lengths of the nanowires usuallyrange from 10-100 μm (FIG. 41).

FIG. 41A shows a scanning electron microscope image of synthesizedsilicon nanowires, according to an example embodiment of the presentinvention. The nanowires of FIG. 41A have diameters on the order of tennanometers, and length extending up to tens of micrometers. The scalebar shown in FIG. 41A is 5 μm in length. FIG. 41B shows alattice-resolved transmission electron microscope image of individual Sinanowires, according to an example embodiment of the present invention.The example nanowires of FIG. 41B have a single crystalline core withcontinuous lattice along their whole length, and an amorphous oxideover-layer, which can be controlled synthetically.

The extended longitudinal dimension and reduced lateral dimension makesthe nanowires the smallest dimension materials for efficient transportof electrical carriers. In addition, the nanowires can be flexiblymanipulated in solution and assembled onto substrate using a electricfield or microfluidic flow approach, and thus enabled the demonstrationof a variety of nanoscale electronic and optoelectronic devices anddevice arrays, including single nanowire field effect transistors(FETs), crossed nanowire FET, and a series of logic circuits such aslogic OR, AND, NOT, NOR gates, and logic half adder circuits and memoryarrays, as well as light emitting diodes, photodetector and highlysensitive chemical/biological sensors.

In particular, studies on single nanowire FET has demonstrated fieldeffect mobility up to 1500 cm²/V·s for Si nanowires, ˜1000 cm²/V·s forGaN nanowires and ˜4000 cm²/V·s for n-type InP nanowires, all comparableor superior to their single crystal counterparts with a similar dopingconcentration. The high mobility value observed in the nanowirematerials highlights the high quality of this new class of materials. Inaddition, these observed mobility values are believed to represent onlya low value for the nanowire materials because little attention has beenpaid to the surface passivation etc., and recent studies showed themobility value can be significantly increased by passivating thenanowire surface. Studies suggest that the scattering events can besignificantly suppressed due the quantum mechanical properties in amodulated doped one-dimensional wire. For example, theoreticalcalculations have predicted mobility of 3×10⁸ cm²/V·s for selectivelydoped GaAs nanowires. Thus it is possible to achieve extremely highcarrier mobility if the dopants are separated from the conductingchannel (e.g., molecular doping from the surface of the nanowires ordoping from the shell in a core-shell nanowire structure).

In summary, nanowires represent a building block for high mobility thinfilm transistors. A randomly oriented nanowire thin film has carriermobility comparable to polycrystalline thin film materials, and anoriented nanowire thin film exhibits mobility value comparable orsuperior to single crystal materials.

TFTs are critical to the development of many modern electronictechnologies. Currently, research and development on TFTs is driven byflat-panel display (FPD) market, dominated by active matrix liquidcrystal displays (AMLCD). A new TFT technology—true silicon on cheaplarge area glass or plastic substrates could revolutionize current FPDtechnology, and open the door for new industries producing new types ofelectronic devices. TFTs incorporating nanowire thin films, according tothe present invention, makes these previously unattainable goalspossible.

Described herein are thin film transistors (TFTs) based on orientedsemiconductor nanowires, and formed on inexpensive glass or flexibleplastic substrates, having performance comparable to that of transistorsfabricated from single crystal silicon:

-   -   Field effect mobility: 1500 cm²/V·s.    -   I_(on)/I_(off): 10⁷.    -   Threshold: <2.5 V.

Conventionally, nanomaterials are used to reduce the size ofelectronics. Embodiments of the present invention, however, usenanomaterials to make electronics faster and/or larger. Although themobility of individual nanowires is high, a single nanowire is unlikelyto be able to provide enough current-density required formacroelectronic applications. To take advantage of the high mobility ofnanowires, transistors are fabricated from oriented nanowire thin filmssuch that many nanowires, including hundreds or thousands of nanowires,span between electrodes (e.g., between source and drain electrodes).This enables high mobility and high current density transistors onlarge, flexible substrates.

FIG. 42 shows a flow diagram of a process for synthesizing andimplementing high mobility nanowire thin film transistors, according toan example embodiment of the present invention. High qualitysingle-crystal nanowire materials are synthesized at high temperature,and are then aligned on a desired substrate to form an oriented nanowirethin film. This can be further subjected to lithography processes toform thin film transistors with the conducting channel parallel to thewire axis. A single crystalline conducting channel along the nanowirelength ensures high mobility for the resulted TFT.

In this approach, silicon nanowire core-shell structure with a singlecrystalline core and dielectric overcoating (shell) can be fabricated,such as shown in FIG. 43, according to an example embodiment of thepresent invention. The nanowires are first synthesized by using therecently developed gold nanoparticle catalyzed chemical vapor deposition(CVD) method and subsequent direct oxidation. The method is applicableto a variety of semiconductor nanowires including silicon (Si) andgallium arsenide (GaAs). Silicon nanowires are referred to herein forillustrative purposes. The chemically synthesized nanowires aresuspended in a solvent, such as alcohol, allowing for subsequent processand manipulation. From these solution suspended nanowires, a monolayernanowire thin film is prepared with wires oriented substantially inparallel on a substrate. Finally, metal contacts for source, drain andgate can be applied through photolithography process to yield nanowireTFT arrays with the conducting channel parallel to the wire axis.

The present invention provides a fundamentally new strategy towards highperformance thin film transistors, and brings in a variety of technicalinnovations and process, performance advantages:

Single crystalline conduction channel: In the present TFT device,multiple nanowires reside in parallel all the way from source to drain(like a log bridge) to provide a single-crystal conducting channel forcarriers. This leads to high carrier mobility comparable to bulk singlecrystal materials, which is impossible to achieve with amorphous orpoly-silicon materials. This is largely due to extensive trapping statesnear the grain boundary which lead to significantly carrier depletionnear boundary and grain boundary scattering for those materials (seeFIGS. 44A-44C).

FIGS. 44A-44C shows views of thin film transistors (TFTs) fabricatedfrom amorphous silicon (FIG. 44A), polysilicon (FIG. 44B), and analigned nanowire thin film (FIG. 44C). In both the a-Si and poly-Sibased technologies, electrical carriers experience multiple grainboundary scattering and thus limit the achievable carrier mobility (˜<1cm²/V·s for a-Si and <100 cm²/V·s for poly-Si). On the other hand, inthe nanowire based technology of the present invention, electricalcarriers transport across the TFT channel along multiple singlecrystalline paths, and thus enable TFTs with carrier mobility approachsingle crystal materials (1000 cm²/V·s).

Off-substrate high temperature process: Semiconductor nanowires and gatedielectrics are prepared off substrate at high temperature and are thenapplied to the substrate at room temperature. Therefore thermalproperties of the substrates will not be a limiting factor for hightemperature process. Thus, this allows for high quality crystallinematerial and gate dielectrics, which are critical for high performancereliable device function. In addition, by incorporating an extremelythin gate-dielectric shell around each individual nanowires, rather thana layer of extrinsic gate-oxide over the device, processing can bedramatically simplified, while reducing the required turn-on voltage dueto the extremely thin and near-perfect nature of the shell.

Solution processibility: Unlike a bulk semiconductor wafer, nanowirescan be suspended in solution and then deposited and secured ontovirtually any substrate over a large area. Thus, high performancesemiconductor materials on many technologically important substratetypes (e.g., plastic, glass) are enabled. This further makes possibleroll-to-roll production of high performance electronics via ink-jet orscreening printing technology.

Mechanical flexibility: Due to an extremely small diameter and largeaspect ratio (>1000), nanowires possess superior mechanical flexibilitywith a radius of curvature as small as 10 micrometer, for example. Bydepositing a dense film of mechanically flexible oriented nanowires ontoa large, flexible substrate, the resulting structure has superiorelectronic performance comparable to a bulk single crystalsemiconductor, and can extend over an arbitrarily large area and be asflexible as a fabric. In addition, the mechanical flexibility ofnanowires enables formation of high performance electronics onrelatively rough substrates, such as most plastics, where the use ofamorphous or poly-silicon thin film is near impossible.

Miniaturized device dimension: The intrinsically small diameter andlarge length of the nanowires allows facile control of TFT channel widthand length. Transistors can be achieved with reduced dimensions, whichis limited in the case of polysilicon device. In addition, the intrinsichigh mobility of nanowire materials allows transistors to be formed withreduced size, while still maintaining a functional current level, andallows high density integration on large area substrates that is notpossible with traditional amorphous silicon or polysilicon materials.

Process applicable to other high mobility materials such as GaAsnanowires: The capabilities of silicon nanowires can be extended andapplied to other intrinsically high mobility materials such as III-Vgroup materials including GaAs and InAs nanowires. Thus ultra-highmobility materials are possible, and many novel applications areenabled. Even higher mobility is possible by taking advantages ofpotential quantum effect—ballistic conduction of nanowires, as furtherdescribed elsewhere herein.

Nanowire synthesis: In some embodiments, nanowires with uniform physicaldimension and chemical dopant distribution can be used to achievereliable and reproducible TFT device behavior. Such control has beendemonstrated in very small scale within a 1 inch tube furnace. 8-inchsemiconductor tube furnaces exist that are capable of producingnanowires at large scale. Control of uniformity over the large volume ismore difficult than over a smaller volume. The control diameters anddiameter distributions of silicon nanowires are determined by thediameters and diameter distributions of gold colloids. Commerciallyavailable gold colloids can be used. The length of nanowires depends onthe growth condition—temperature, vapor pressure and duration of growth.These issues, and issues of crystallinity and doping concentration, canbe addressed by varying and fine tuning the growth conditions.

Dielectric oxide/nitride coating: The quality of a gate dielectriccoating is important to nanowire TFT performance. The technology forformation of a high quality gate dielectric is present in planarsemiconductor technology. However, forming a pin-hole free gatedielectrics with uniform thickness around the surface of nanowireswithout a defined crystallographic direction is technically challenging.This issue can be addressed from different perspectives. A process canbe used to generate a uniform silicon oxide coating less than ˜2 nm byslow thermal oxidation of silicon nanowires immediately following theirgrowth. A key to controlling the quality of the oxide coating isnanowires having a smooth and consistent surface structure. A lowoxidation process aids in avoiding hot spots and generating pin-holefree thin coatings. Alternatively, plasma assisted direct nitridation ofsilicon oxide coated nanowires can be used for generation of oxynitrideor nitride coating on the nanowire surface.

Surface states and trapped charges: Due to its high surface and bulkatomic ratio, surface states, trapped charges and dangling bonds affectnanowires, which could significantly limit device performance. A numberof strategies can be used to minimize the surface states, includingdirect thermal annealing under an inert atmosphere or underhydrogen/forming gas atmosphere, and annealing in hydrogen plasma,followed by a thermal annealing. These and other strategies aredescribed in further detail above.

Large area nanowire thin film deposition: The development of a scalableapproach for the preparation of oriented nanowire thin film on largearea substrate is important to the eventual successful implementation ofthis new technology. For example, a flow alignment process orLangmuir-Blogette film approach can be used to place a monolayer ofnanowires on a glass or a plastic substrate.

Ohmic contact: Making reliable Ohmic contacts with nanowires isdifficult due to small contact areas and complicated interface states.Interface chemistry and physics between metal contacts and silicon areimportant technical areas regarding Ohmic contacts. A key to success isthe precise control of the metallization process and surface cleaningprocess prior to metallization. Three possible metallization schemes canbe used—Ti—Au, Ni and Al by e-beam evaporation. Various furtherprocesses, including ion gun cleaning, or HF etching can be employed toremove the surface dielectrics prior to metallization of source-drainelectrodes.

The present invention allows for a revolutionary advance in large areaelectronics, and offers a new generation of electronic devices byproviding a thin film TFT with performance comparable to inorganicsingle crystal semiconductor materials, as well as size and flexibilityof plastic electronics.

TFT nanowire transistors can be manufactured with performancecharacteristics approaching those of transistors fabricated fromtraditional single crystal silicon, most significantly on very largeglass or plastic substrates, which will enable ultra-large scale highdensity integration and provide a true silicon on the plastictechnology. The potential applications of this technology is very broad,including incorporating nanowire TFTs into liquid crystal displays(LCD). A nanowire TFT has a much smaller foot-print than that of a-SiTFT, allowing for an increase in pixel density—e.g., a ultra-highdensity display. The smaller TFT also blocks less light and has a higheraperture ratio. With a nanowire TFT, periphery driver circuits can besimultaneously integrated on the edge of the glass substrate,significantly simplifying the fabrication process and lowering the cost.Nanowire TFT are applicable to microdisplays, digital projectors andhigh density imaging devices that require very high pixel densities.Furthermore, a true silicon-on-plastic technology enables development oflight, high information density electronic devices. For example, thepresent invention enables computing and displaying on a single sheet,and enables wearable electronics, which can be especially important forpolice, emergency officers working in the field, soldiers in thebattlefield and space and remote exploration.

In a broader sense, the present invention allows engineers to developelectronic materials with the performance and materials characteristicsof any industrially-relevant semiconductor material on any substratematerial. This technology enables engineers to design the functionalcharacteristics of an electronic material (i.e., the conductiveproperties, doping, mobility and turn-on-voltages) completelyindependently from the structural characteristics (i.e., itsflexibility, shape, size and process characteristics). The selection ofthe physical characteristics of the semiconductor nanowire component(e.g., composition, diameter, length, crystallinity and density)determine the electronic performance; the nature of the substrate, whichcan be selected completely independently, the physical performance.Together, the high performance, easy processibility and potentiallylow-cost of nanowire thin film produces a versatile flexible electronicsplatform, and is useful not only for driving circuits for high-densitydisplay and micro-displayer array, radio frequency identification tags,large area biosensor but also for many new application such as smartcards, logic and memory for wearable computers on plastic and for manymore important applications yet to be identified.

Three development stages are described as follows: (1) silicon nanowiresynthesis; (2) oriented nanowire thin film deposition; and (3) nanowirethin film transistor (TFT) fabrication.

(1) Silicon Nanowire Synthesis

Step 1: A scalable process for silicon nanowire synthesis is describedbelow.

An example nanowire suspension solution, nanowire type, and nanowireconcentration is provided, for illustrative purposes: 1 mg/100 cc inalcohol, p and n-doped 60 nm diameter silicon nanowires, having acore-shell structure. The nanowire core is single-crystal silicon. Thenanowire shell is ˜2 nm thick pin-hole-free silicon oxide or oxynitridecoating, length of ˜20-50 μm.

The synthesis uses a gold nanoparticle catalyzed CVD process. Apre-determined precursor gas mixture, SiH₄ and B₂H₆ or PH₃ in He passesover the catalyst-gold particles deposited on an oxide coated siliconsubstrate at a total pressure between 20-50 Torr, while the goldnanoparticles are heated up to a temperature ˜450° C. Upon contact withthe gold nanoparticles, SiH₄/B₂H₆ is decomposed, and Si and B atomsdiffuse into the gold nanoparticle and generate a liquid alloy droplet.Once an over-saturation is reached, Si/B atoms precipitate out andinitiate a nanowire growth. Continuously supplied SiH₄ and B₂H₆ allowsthe nanowires to continue growing until intentionally terminated, or a“death” caused by local condition change. The quality of the nanowiresis dependent on the quality of gold nanoparticles, control of goldnanoparticle distribution on the substrate and growth conditionincluding temperature, ratio of SiH₄ to B₂H₆ or PH₃, partial pressure ofthe SiH₄, and resident time of precursor gases in the reactor.

In the current example embodiment, the growth is accomplished using acomputer controlled 8″ semiconductor furnace. A 4″ silicon oxide coatedsilicon wafer is used as the substrate.

A process for uniformly deposition of gold nanoparticles on 4″ wafers isdescribed.

Commercially available 60 nanometer diameter gold colloids are used. Thetarget is to achieve a uniform deposition of gold nanoparticle withdensity between 2-4 particles per micrometer sq. A key is minimized goldparticle cluster formation. The clusters can result in undesired largerdiameter nanowire growth. Spin coating and self assembly methods can beexplored for the deposition.

Spin coating is a fairly straightforward process. A deposition densitycan be controlled through variation of the gold particle concentrationin the precursor colloids, manipulation of surface chemistry of thesilicon wafer and changing the spin speed. A drawback of spin coatingcan be low utilization efficiency of gold colloid solution. A recyclingprocess in the production stage can be used if warranted.

Self assembly involves some use of established chemistry. The surface of4″ silicon oxide coated wafer is functionalized with either(3-aminopropyl)-trimethoxysilane (APTMS) or(3-mercaptopropyl)-trimethoxysilane (MPTMS), then contacted with 60nanometer gold colloid solution. The gold particles are assembled on thesurface. The difference between two different chemistries are compared,and the possibility of controlling the density of gold particles bycontrol of the contact time and gold particle concentration in thecontact solution may be used.

Step 2. Optimize nanowire growth condition: Growth parameters need to beoptimized including ratio of SiH₄ to B₂H₆ or PH₃, partial pressures ofSiH₄, and B₂H₆ or PH₃, total pressure, gas flow rate, growthtemperature, and growth duration. The diameter distribution of siliconnanowires can be determined by that of gold nanoparticles. Commerciallyavailable 60 nanometer gold colloids can have a diameter distribution of±10%. The same distribution is our target for the nanowires. Goldnanoparticles can be split into smaller ones resulting in smallerdiameter nanowire growth, depending on the growth condition. The growthcondition can be optimized to minimize this event. Given a growthcondition, the length of nanowires can be controlled by varying durationof the growth. Crystallinity of silicon nanowires and dopantconcentration are also growth condition dependent. They can be optimizedand controlled together with other important nanowire characteristics.

Another issue is thermal decomposition of SiH₄ and B₂H₆ under the growthcondition in order to grow high quality silicon nanowires. Thisdecomposition can generate unwanted silicon nanoparticles in theproduct. In their small scale growth, the thermal decomposition can noteasily be eliminated, but can be minimized by varying the condition ofgrowth

Step 3: Process for dielectric thin film coating: The quality ofdielectric coating is a key factor in determining the performance ofnanowire TFT. Methods and underpinning scientific principles for highquality dielectric coating have been well established for planner singlecrystal silicon, lately for amorphous and polysilicon. In general,methods can be categorized into direct oxidation/nitridation for siliconoxide/nitride dielectrics and CVD deposition for all kinds of dielectriccoatings. A unique structural nature of silicon nanowires leads toselection of direct oxidation/nitridation over the deposition method.However, the deposition method can be used as an alternative.

A silicon dioxide thin coating with a thickness of ˜2 nm can be used.The direct oxidation of 60 nanometer diameter silicon nanowires cancarried in the nanowire growth furnace. After nanowire growth isterminated, the reactant gas mixture can be depleted from the reactiontube and replenished with a mixture of oxygen (5%) and helium at atemperature below 150° C. The temperature of the furnace can then beslowly raised to a number between 300° C. and 800° C. The oxidationtemperature together with a ratio of oxygen to helium, partial pressureof oxygen, and duration of oxidation determines the thickness of siliconoxide generated. These conditions can be optimized until a thickness of˜2 nm is obtained. A slow oxidation is desired in order to minimizedefects, dangling bonds, which can result in trapped charges and states.

If the performance of silicon oxide thin coating is unsatisfactory,direct nitridation of silicon oxide coated nanowires to generateoxynitride coating can be used. The higher dielectric constant madeoxynitride and nitride a more attractive coating. A plasma assisteddirect nitridation method using gases of NO or NH₃ can be used.

Step 4. Process for removing surface states and trapped charges: Surfacestates and trapped charge can be a serious issue for nanowires becauseof their high surface to bulk atom ratio. Processes for dealing withthis issue are known in the semiconductor industry for plannerprocesses, which are applicable to nanowires. First we can anneal inhydrogen, using a single nanowire device test as a feedback forcondition optimization.

Step 5. Prepare the nanowire suspension in alcohol: After being coatedwith a dielectric coating and being annealed, silicon nanowires can beremoved from the 4″ (or other size) wafer and suspended in alcohol byultrasonication. Nanowires may agglomerate and precipitate out. Asurfactant like Triton X-100 can be used for stablization.

Step 6. Nanowire characterization: The structural and electricproperties of nanowires can be characteized. SEM and AFM can be used forcharacterization of their length and diameter distribution, highresolution TEM for thickness and uniformity of dielectric thin filmcoating, transport measurement, EFM, scanning gate AFM for theirelectric properties. The results of characterization can be feed backfor fine tuning of synthetic procedure and conditions.

(2) Oriented Nanowire Thin Film Deposition

A scalable method for oriented nanowire monolayer thin film depositionis described below.

Result: An oriented monolayer 60 nm silicon nanowire film on a 4″×4″glass or plastic, such as polystyrene.

Oriented nanowire arrays are critical to ensuring a single crystalconducting channel between source and drain of the nanowire TFT, whichmake high field effect mobility possible. To obtain a highly orientednanowire thin film over a large area, two basic approaches can be usedas parallel strategies, fluidic flow approach and Langmuir-Blodgett filmapproach.

Step 1. Generic chemistry for nanowire surface modification: The goal ofthis task is to develop a generic set of processes for modifying asilicon nanowire surface to facilitate stable suspension of nanowires ina non-polar solvent, which is necessary in order to practice theLangmuir-Blodgett film approach. This can be achieved using a genericsilicon oxide surface chemistry. We can use alkyl-trimethoxysilane toattach hydrophobic alkyl group, such as octyl group on the surface ofnanowires. This can make nanowire suspendable in organic solvent, suchas octane. These surface groups may have a detrimental effect on theelectronic properties of nanowires. Methods for removing the organicmolecules (e.g., oxygen plasma or ozone cleaning process) from a siliconnanowire surface subsequent to nanowire thin film formation can bedeveloped when necessary.

Step 2. Process for substrate surface treatment: Surface chemistry ofsubstrate is important to adhesion of nanowires to the substrate. Incase of glass substrates, nanowires intrinsically adhere to it nicelybecause of similar surface chemistry of glass and silicon oxide coatednanowires. A routine clean and a quick plasma oxygen plasma etch can bedone. For hydrophobic plastic substrate, we can first do an oxygenplasma oxidation, then attach a monolayer of 3-aminopropyl-group to thesurface using 3-aminopropyltrimethoxysiline. The rigid polystyrene sheetcan be tested first. If successful, the flexible film, like a piece ofpolypropylene, can be used next.

Step 3. Fluidic flow alignment: For fluidic alignment, we use nanowiressuspended in alcohol. The fluidic flow approach has been applied toalign nanowires on a scale with width of hundreds of micrometer andlength of a few inches. In principle, the fluidic flow alignment can beextended to very large area just like logs in a river. In order toachieve alignment over large area, a fluidic channel with lateraldimension comparable to the substrate size can be used. The height ofthe channel can be controlled to be less than 500 μm so that a majorportion of the nanowire solution is proximate to the substrate, and theshear flow near the surface of the substrate enables the alignment ofthe nanowires along the flow direction. FIG. 45 shows a diagram of afluidic cell for aligning nanowires over a large area, according to anembodiment of the present invention. An detailed example method andsystem for such a fluidic cell alignment approach is described abovewith reference to flow mask 1500, as shown in FIGS. 15A and 15B. Theentrance and outlet of the fluidic cell is carefully designed to ensurea uniform flow along and across the whole flow channel.

Various nanowire solution concentration and flow times can be used tocontrol a nanowire surface density/coverage on the substrate. Whendesired, the substrate can also be functionalized to enhance thecomplementary interaction between the substrate and the nanowires toachieve higher surface coverage. A systematic study can be conducted toenable reproducible nanowire deposition on surface. The surface coveragecan be studied with an optical microscope and/or scanning electronmicroscope, and a rational statistical approach can be developed toquantitatively characterize the surface coverage. These studies can befirst conducted on glass substrate and can be implemented to surfacefunctionalized plastic substrate.

Several critical issues should be noted and carefully controlled: 1) arubber stamp (polydimethylsiloxane, PDMS) fluidic channel used for smallscale alignment may not be applicable at inch- to tens-of-inch-dimensionbecause the channel can potentially collapse in the middle part due tothe flexible nature of PDMS. To overcome this issue, a solid channelusing glass or stainless steel can be used. The perimeter of the channelcan be sealed using either o-ring or coating a thin layer of PDMS. 2) Atsuch a large dimension scale, the flow across and along the wholechannel may not be uniform which is not desired for uniform nanowiredeposition. To enable uniform flow, particular attention should be paidto design and engineering the fluidic channel entrance and outlet. Greatcare is also taken with design solution delivery scheme. A programmableautomatic syringe pump can be used to ensure a constant solutiondelivery rate. There is high technical risk in terms of achieving auniform nanowire deposition on surface. For example, it is likely tohave much higher nanowire density in the area near the entrance of thechannel than the area near the outlet, which is often observed inmicro-channel fluidic alignment without careful design of the channelentrance. The density variation can be compensated by alternativelyreversing the flow direction during the alignment process.Alternatively, a Langmuir-Blodgett film technique can be used, which cangive uniform alignment over a large area. However, the results fromfluidic flow alignment can still be used for initial test on devicefabrication and characterization.

Step 4. Langmuir-Blodgett Film: In order to achieve a uniform alignmentover a large area, a large scale assembly approach based onLangmuir-Blodgett (L-B) film can be used. Langmuir-Blodgett alignmenthas been used to form thin films of nanoparticles and aligned nanorods.This approach can be extended to the alignment of nanowires to producean oriented nanowire thin film. FIG. 46 shows a diagram illustrating thealignment of nanowires over a large area using a Langmuir-Blodgett film,according to an example embodiment of the present invention.

In this approach, the nanowires are first functionalized and suspendedin non-polar solvent (Step 1 above). Such a non-polar nanowiresuspension is then transferred onto water surface in an L-B trough. Atsufficient low-densities, the nanowires form an isotropic distributionwith random orientation. As the surface is compressed, it becomesincreasingly difficult for the wires to point in random directions andthe nanowires undergo a transition to a more ordered anisotropic phasewith uniaxial symmetry to have a nematic or smectic phase (see FIG. 46).Indeed, this has been observed in Monte-Carlo simulations and in realexperiment for the case alignment of thin film nanorods (e.g., aspectratio=length/diameter<10). Thus it is possible to achieve alignment ofnanowires on water surface over large area. In addition, directionalcapillary force and van der Waals attraction between nanowires furtherenhance the parallel alignment of the nanowires and the formation of anoriented nanowire thin film. Due large significantly larger aspect ratioof the nanowires (e.g., >500), the nanowires may experiencesignificantly larger resistance to rotating from a random orientation toparallel to each other. This potential problem can be solved by inducingsome pre-alignment prior to surface compression, for example. A numberof strategies can also be employed to achieve this goal. For example, aflow process can be combined to achieve some pre-alignment. Anelectrical field can also be applied to enhance the alignment of thewires. The aligned nanowires on water were then transferred onto adesired substrate. The nanowire density can be controlled by the ratioof surfactant and nanowires and by the amount of surface compression.After an aligned nanowire thin film is formed on the water surface, itcan be transferred onto any substrate. Different transfer protocols maybe used to avoid disturbing the alignment during the transferring. Thesurface coverage can be characterized using a similar approach describedabove.

(3) TFT Fabrication and Characterization.

Silicon nanowire TFTs with field effect mobility of ˜1000 cm²/V·s,on/off current ratio of >10⁷, and threshold voltage of <2.0 V arefabricated in the current example, as described below.

Result: A 1000×1000 array of silicon nanowire TFT on a glass substrateand a polystyrene substrate.

Despite that FETs has been demonstrated using a single nanowire as theconducting channel, the application of these FETs has often sufferedfrom a large variance from device to device. This variance may be due toa lack of synthetic control, a reliable way to make ohmic contact, and alarge number of surface trap states. Achieving reliable and controllableelectrical characteristics is critical for any real application of TFTs.In order to achieve high level control of device characteristics, theelectronic properties of individual nanowires must be highlyreproducible and controllable. Electronic quality control of thenanowires can be characterized and optimized using a single nanowire FETstructure. With well controlled electronic properties, nanowire thinfilm transistor devices can be fabricated and characterized on differentsubstrate including glass and plastics.

Step 1. Process for a Reliable Metallization Process using SingleNanowire FET:

A single nanowire transistor FET can be used as a test vehicle todevelop a reliable metallization process. A single nanowire FET devicecan be fabricated on SiO₂/Si surface using either electron-beamlithography or photolithography. The substrate silicon can be a globalback gate, and two metal electrodes can be used as source and drainelectrodes (FIG. 47). Prior to metallization, appropriate surfacecleaning procedure can be taken to remove the oxide from the nanowiresurface and ensure a good contact between the nanowire and contactmetal. Planar silicon technology can be used as a reference point forchoosing appropriate contact metal and process protocol. Variousstrategies, including ion gun cleaning, or HF etching can be employed toremove the surface dielectrics prior to metallization of source-drainelectrodes. Different metallization recipes (E.g., Ti/Au, Ni/Au, Al) canbe tested and optimized using either electron-beam evaporation orsputtering process. Attention is paid to the surface treatment (viathermal annealing and hydrogen plasma annealing) of nanowires and itseffects on the electronic properties. The device behavior can becharacterized using a semiconductor analyzer. Various measurementconfiguration including gate-dependent two-terminal measurement andfour-terminal measurements, as well as electric force microscopy can beemployed to characterize the device behavior. The device structure canbe stringently modeled theoretically to derive all the key transistorparameters including carrier concentration and mobility, thresholdvoltage, on/off ratio etc. The results from the electrical test andtheoretical modeling can further be feed back to optimize metallizationprocess until a reliable process is obtained. This can be a standardmetallization process for the fabrication nanowire TFT. The singlenanowire FET is also used for the nanowire qualification test. Adatabase can be constructed in terms of synthetic conditions and theelectronic parameters of nanowires, the database wcan be further used toguide a more controllable synthesis and device fabrication process.

FIG. 47 shows plan and perspective views of a single nanowire fieldeffect transistor, according to an example embodiment of the presentinvention. The single nanowire FET is used as the basic device geometryto characterize and optimize the electrical transport properties ofindividual nanowires. The perspective view shown in FIG. 47 shows ascanning electron microscope (SEM) image of a typical device. In thisdevice a silicon substrate can used as the back gate, and two metalelectrodes can be used as the source-drain contacts

Step 2. Global-back-gated Nanowire TFT: The goal of this task is todemonstrate and characterize nanowire TFT fabricated from nanowire thinfilm with different surface density using a similar device structuredescribed above. A reliable protocol identified for the fabrication ofsingle nanowire device can be applied to nanowire thin film transistors.TFT devices can be fabricated using nanowire thin films with differentsurface density to achieve individual TFT devices with variable numberof nanowires bridging the source and drain electrodes. A semiconductoranalyzer can be used to characterize the devices behavior such ascurrent level, on/off ratio, threshold voltage and gate leakage currentas a function of nanowire surface density, and device behavior can betheoretically modeled to calculate the critical device parametersincluding the mobility value. The modeling can in turn be used to directthe design of device structure to achieve desired device function. Thesestudies can be carried out on SiO₂/Si substrate using silicon back asthe back gate, as this is an relatively easy way for device fabricationand modeling. At the end of this task, a reliable protocol can be formedto fabricate TFTs with variable nanowire surface density andcontrollable device behavior.

Step 3. Local-gated nanowire-FET on glass and plastics: Fabrication ofnanowire TFTs and TFT arrays on glass and plastic substrate aredescribed, for example, targeting mobility around ˜1000 cm²/V·s. A TFTusing locally patterned gate electrodes is used. Locally-gated TFTstructure are fabricated using Si/SiO₂ or Si/Si₃N₄ core-shell nanowires,where SiO₂ or Si₃N₄ shell can be used as the gate dielectrics, and anadditional metal electrode can be employed as the gate electrode (FIG.48).

FIGS. 48A and 48B show perspective views of locally gated nanowire thinfilm transistors, according to embodiments of the present invention.FIG. 48A shows a staggered structure with the gate formed beneath thenanowire thin film, and the source drain electrode from on the top ofthe nanowire thin film with the dielectric over-layer removed. FIG. 48Bshows a TFT structure with all the contacts formed on the top of thenanowire thin film.

A bottom contact and top-contact gate configuration can both be testedto ensure the lowest switching voltage, largest on/off ratio, and lowestleakage current. All these tests can be done on first on glass substrateand then be applied to plastics. Resultingly, individual nanowire TFTscan be fabricated on glass and plastic substrate with controllabledevice characteristics (carrier mobility ˜1000 cm²/V·s, controllablethreshold (<2.5 V), current level (1 μA-1 mA) and on/off ratio (>10⁶)).

Step 4. Integrated nanowire TFT arrays: As a reliable protocol isidentified for making individual local-gated TFTs on glass and chosenplastics with desired device characteristics, it can be applied tofabricate an integrated array of TFT devices on a 4×4″ glass and plasticsubstrate. Multi-level photolithography can be used to pattern thenanowire thin film and to form the source-drain, gate electrode arrayfor an integrated TFT array. Particular attention may be paid to theexact device structure design and device fabrication process design. Themature technology of amorphous silicon and polysilicon TFTs can be usedas a reference point for such design. In the end, a 1000×1000 array ofnanowire TFTs can be fabricated and further be configured into logiccircuits to produce logic function.

Electrical Devices Including Nanowire Layers, and Nanowire LayersCombined with Single-Crystal, Amorphous, and PolycrystallineSemiconductor Materials

In another aspect of this invention, electrical devices can be formedusing a structure that includes multiple nanowire thin film layers. Inother words, multiple layers of nanowire thin films, such as the variousembodiments described above, can be stacked to create devices.

For example, in an embodiment, a first plurality of nanowires aredeposited on a substrate to form a first nanowire thin film layer. Asecond plurality of nanowires are deposited on the first nanowire thinfilm layer to form a second nanowire thin film layer. Any number ofadditional pluralities of nanowire can also be deposited on precedingnanowire thin film layers to form additional thin film layers in astack.

For illustrative purposes, referring to a two-thin film layerembodiment, nanowires of the first and second layers can be differentlydoped. For example, the first layer can include nanowires that arep-doped, and the second layer can include nanowires that are n-doped.Thus, junctions, such as p-n junctions, are thereby formed at the crosspoints/intersections between the nanowires of the first and second thinfilm layers.

Contacts can be formed to create electrical devices based on theproperties of the junctions. For example, in a diode embodiment, a firstcontact can be formed that is coupled to nanowires of the first thinfilm and a second contact can be formed that is coupled to nanowires ofthe second thin film. Thus, a two-terminal p-n diode can be formed.Three- and other-terminal number devices, such as transistors, can beformed in a similar manner.

Note that nanowires of the first thin film layer are preferably alignedparallel to each other, and nanowires of the second thin film layer arepreferably aligned parallel to each other. However, in alternativeaspects, the nanowires of the first and/or second thin film layers canbe randomly oriented.

In another embodiment, an electrical device is formed that includes ahybrid nanowire-single crystal semiconductor structure. For example, asingle-crystal semiconductor strip/thin film is formed. For example, thesingle crystal strip can be formed by etching a wafer, such as asilicon-on-insulator (SOI) wafer. Furthermore, the single-crystalsemiconductor strip/thin film can be formed to have any shape or size asrequired by a particular application. A plurality of nanowires aredeposited on the top of the strip. Junctions, such as p-n junctions, areformed at the cross points between the nanowires and the single crystalsemiconductor strip. For example, the single crystal semiconductor stripcan be doped a first way (either n- or p-doped) and the nanowires can bedoped a different way (e.g., p- or n-doped). Thus, p-n junctions can beformed at the intersections of the strip with the nanowires.

Contacts can be formed to create electrical devices based on theproperties of these junctions. For example, in a diode embodiment, afirst contact can be formed that is coupled to the single crystalsemiconductor strip and a second contact can be formed that is coupledto the nanowire thin film. Thus, a two-terminal p-n diode can be formed.Three- and other-terminal number devices, such as transistors, can beformed in a similar manner.

The nanowires of the plurality of nanowires are preferably alignedparallel to each other, but can alternatively be randomly oriented.

In another embodiment, an electrical device is formed that includes ahybrid nanowire-amorphous/polycrystalline semiconductor structure. Forexample, an amorphous or polycrystalline semiconductor thin film isdeposited on a substrate. A plurality of nanowires are deposited on thethin film pattern. Junctions, such as p-n junctions, are formed at thecross points/interasections between the nanowires and theamorphous/polycrystalline semiconductor thin film pattern. For example,the amorphous or polycrystalline semiconductor thin film can be doped afirst way (either n- or p-doped) and the nanowires can be doped adifferent way (e.g., p- or n-doped). Thus, p-n junctions can be formedat the intersections of the amorphous or polycrystalline semiconductorthin film with the nanowires

Contacts can be formed to create electrical devices based on theproperties of these junctions. For example, in a diode embodiment, afirst contact can be formed that is coupled to the amorphous orpolycrystalline semiconductor thin film and a second contact can beformed that is coupled to the nanowire thin film. Thus, a two-terminalp-n diode can be formed. Three- and other-terminal number devices, suchas transistors, can be formed in a similar manner.

The nanowires of the plurality of nanowires are preferably alignedparallel to each other, but can alternatively be randomly oriented.

In an example embodiment, light emitting devices can be created usingthese structures. For example, light emitting semiconductor nanowiressuch as those emitting red, green and blue light, can be selected in apredetermined ratio of nanowires. Nanowires selected in thepredetermined ratio can be mixed in a solution. The nanowire mixture isflowed across a single-crystal, amorphous, or polycrystallinesemiconductor strip/thin film. As described above, contacts can beformed to create a light emitting electrical device, such as a lightemitting diode in a two-contact/terminal device. Depending on theselected mixture of light emitting nanowires, any color of light can beemitted by the light emitting electrical device, including white light.

Applications of the Present Invention

Numerous electronic devices and systems can incorporate semiconductor orother type devices with thin films of nanowires, according toembodiments of the present invention. Some example applications for thepresent invention are described below or elsewhere herein forillustrative purposes, and are not limiting. The applications describedherein can include aligned or non-aligned thin films of nanowires, andcan include composite or non-composite thin films of nanowires.

Semiconductor devices (or other type devices) of the present inventioncan be coupled to signals of other electronic circuits, and/or can beintegrated with other electronic circuits. Semiconductor devices of thepresent invention can be formed on large substrates, which can besubsequently separated or diced into smaller substrates. Furthermore, onlarge substrates (i.e., substrates substantially larger thanconventional semiconductor wafers), semiconductor devices formed thereonaccording to the present invention can be interconnected.

The present invention can be incorporated in applications requiring asingle semiconductor device, and to multiple semiconductor devices. Forexample, the present invention is particularly applicable to large area,macro electronic substrates on which a plurality of semiconductordevices are formed. Such electronic devices can include display drivingcircuits for active matrix liquid crystal displays (LCDS), organic LEDdisplays, field emission displays. Other active displays can be formedfrom a nanowire-polymer, quantum dots-polymer composite (the compositecan function both as the emitter and active driving matrix). The presentinvention is also applicable to smart libraries, credit cards, largearea array sensors, and radio-frequency identification (RFID) tags,including smart cards, smart inventory tags, and the like.

The present invention is also applicable to digital and analog circuitapplications. In particular, the present invention is applicable toapplications that require ultra large-scale integration on a large areasubstrate. For example, the thin film of nanowires embodiments of thepresent invention can be implemented in logic circuits, memory circuits,processors, amplifiers, and other digital and analog circuits.

The present invention can be applied to photovoltaic applications. Insuch applications, a clear conducting substrate is used to enhance thephotovoltaic properties of the particular photovoltaic device. Forexample, such a clear conducting substrate can be used as a flexible,large-area replacement for indium tin oxide (ITO) or the like. Asubstrate can be coated with a thin film of nanowires that is formed tohave a large bandgap, i.e., greater than visible light so that it wouldbe non-absorbing, but would be formed to have either the HOMO or LUMObands aligned with the active material of a photovoltaic device thatwould be formed on top of it. Clear conductors can be located on twosides of the absorbing photovoltaic material to carry away current fromthe photovoltaic device. Two different nanowire materials can be chosen,one having the HOMO aligned with that of the photovoltaic material HOMOband, and the other having the LUMO aligned with the LUMO band of thephotovoltaic material. The bandgaps of the two nanowires materials canbe chosen to be much larger than that of the photovoltaic material. Thenanowires, according to this embodiment, can be lightly doped todecrease the resistance of the thin films of nanowires, while permittingthe substrate to remain mostly non-absorbing.

Hence, a wide range of military and consumer goods can incorporate thethin film of nanowires embodiments of the present invention. Forexample, such goods can include personal computers, workstations,servers, networking devices, handheld electronic devices such as PDAsand palm pilots, telephones (e.g., cellular and standard), radios,televisions, electronic games and game systems, home security systems,automobiles, aircraft, boats, other household and commercial appliances,and the like.

Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A method of fabricating nanowires comprising: (A) selecting asemiconductor material; (B) forming a plurality of nanowires from theselected semiconductor material; (C) oxidizing a circumferential surfaceof each nanowire of the plurality of nanowires to create a plurality ofoxidized nanowires; and (D) annealing each oxidized nanowire of theplurality of oxidized nanowires.
 2. The method of claim 1, wherein step(D) comprises: annealing each oxidized nanowire in an H₂ environment topassivate dangling bonds at the interface of an oxidized layer and anon-oxidized portion of each oxidized nanowire.
 3. The method of claim1, wherein the semiconductor material comprises silicon which isoxidized in step (C) to create an oxide layer of SiO₂ on the pluralityof silicon nanowires.
 4. The method of claim 3, further comprising thestep of nitriding the silicon dioxide layer to form an oxynitridecoating on the plurality of nanowires.